How Much Does a Double-Storey Extension Cost? (2026 UK Guide)
A double-storey extension is one of the most cost-effective ways to add substantial space to your home. By building upwards as well as outwards, you maximise the use of your foundation and roof — spreading the most expensive structural costs across two floors instead of one. For growing families in the UK, a well-designed double-storey extension can add a new kitchen-diner downstairs and an extra bedroom or bathroom upstairs, all without losing garden space.
In 2026, the average double-storey extension cost in the UK ranges from £1,800 to £2,500 per m² for a standard-quality build, according to industry cost data from Checkatrade, the Federation of Master Builders (FMB), and the HomeOwners Alliance. For a typical 40 m² double-storey extension (20 m² per floor), you should budget between £72,000 and £100,000 for construction alone, with total project costs — including design, planning, and professional fees — typically reaching £90,000 to £125,000.
This guide provides a complete cost breakdown, explains how location affects pricing, and shares practical strategies to keep your project on budget. If you want to see what is possible for your specific property, get a free AI render from Suddeco — enter your address and receive an instant visualisation, cost estimate, and planning feasibility report in seconds.
Table of Contents
- How Much Does a Double-Storey Extension Cost? (2026 UK Guide)
- Average Double-Storey Extension Cost Per m²
- Cost Breakdown by Size
- Location-Specific Pricing (London vs UK)
- What Is a Double-Storey Extension?
- What Affects the Cost?
- Hidden Costs to Budget For
- How to Save Money on a Double-Storey Extension
- Double-Storey vs Single-Storey Extension
- Planning Permission and Building Regulations
- Do You Need an Architect?
- Get Your Free AI Render
- Frequently Asked Questions
- JSON-LD FAQ Schema
Average Double-Storey Extension Cost Per m²
The cost per square metre is the most reliable benchmark for budgeting any extension. Because a double-storey extension shares the same foundation and roof across two floors, the cost per m² is typically lower than a single-storey extension of the same total floor area. In 2026, Checkatrade and FMB members report the following price ranges:
| Quality Level | Cost per m² (UK Average) | Cost per m² (London) | Cost per m² (South East) | Cost per m² (North/Midlands/Scotland) |
|---|---|---|---|---|
| Budget / Basic | £1,400 - £1,800 | £1,800 - £2,500 | £1,700 - £2,300 | £1,100 - £1,400 |
| Standard / Mid-Range | £1,800 - £2,500 | £2,340 - £3,750 | £2,160 - £3,250 | £1,440 - £2,000 |
| High-End / Premium | £2,800 - £4,000+ | £3,600 - £5,500+ | £3,200 - £4,800+ | £2,200 - £3,200+ |
Sources: Checkatrade 2026 cost guide, FMB members survey, HomeOwners Alliance extension cost guide. Figures include construction costs only. Design, planning, and professional fees are additional.
A standard-quality double-storey extension — featuring a quality kitchen or living space downstairs, a bedroom or bathroom upstairs, good insulation, and standard finishes — typically costs £1,800 to £2,500 per m² in most UK regions. In London, where labour costs and site constraints push prices higher, expect to pay £2,340 to £3,750 per m².
The average cost per m² across the UK sits at approximately £2,150, making a 40 m² double-storey extension (20 m² per floor) roughly £86,000 for construction, with an additional £15,000 to £25,000 for design, planning, structural engineering, and contingency.
For a precise estimate tailored to your property and location, try Suddeco's free AI render tool. Our platform analyses nearby approved planning applications to show exactly what can be built — and what it should cost.
Cost Breakdown by Size
Double-storey extensions are measured by their ground floor footprint, with the first floor matching the same dimensions. Below is a realistic cost breakdown for a standard mid-range finish in the UK average region:
| Ground Floor Size | Total Floor Area | Construction Cost (Min) | Construction Cost (Average) | Construction Cost (Max) | Total Project Cost (Inc. Fees) |
|---|---|---|---|---|---|
| 15 m² | 30 m² | £54,000 | £64,500 | £75,000 | £70,000 - £95,000 |
| 20 m² | 40 m² | £72,000 | £86,000 | £100,000 | £90,000 - £125,000 |
| 25 m² | 50 m² | £90,000 | £107,500 | £125,000 | £110,000 - £155,000 |
| 30 m² | 60 m² | £108,000 | £129,000 | £150,000 | £130,000 - £185,000 |
| 40 m² | 80 m² | £144,000 | £172,000 | £200,000 | £170,000 - £245,000 |
What "Total Project Cost" Includes
The total project cost column above includes: - Construction (the figures in the first three columns) - Architectural design (£3,750 - £7,500 for a typical £75k-£100k build, typically 5-10% of construction cost) - Planning permission (£206 for householder applications) - Building regulations approval (£200 - £1,000) - Structural engineer (£500 - £2,000) - Party wall agreement (if applicable, £700 - £2,000) - Contingency (10-15% of construction cost)
For a 40 m² double-storey extension (20 m² per floor) at the UK average, the minimum total cost is approximately £90,000, the average is £105,000, and the maximum for a premium finish is £125,000.
Location-Specific Pricing (London vs UK)
Location is the single biggest variable affecting double-storey extension costs. Labour rates, material availability, site access, and local demand all create significant regional differences.
London
London commands the highest extension costs in the UK. The Federation of Master Builders (FMB) reports that London builders charge 30-50% more than the UK average, driven by: - Higher labour costs (skilled trades in London earn £250-£400/day vs £180-£280/day elsewhere) - Limited site access and parking restrictions - Higher material delivery costs - Strong demand for extra bedrooms in family homes close to good schools
London double-storey extension cost per m²: £2,340 - £3,750 (standard) | £3,600 - £5,500+ (high-end)
Total project cost for a 40 m² double-storey in London: £105,000 - £150,000
South East (Surrey, Kent, Sussex, Hampshire, Berkshire)
The South East sits just below London in pricing, with a 20-30% premium over the UK average. Demand for double-storey extensions is exceptionally high in commuter belt areas where families want extra bedrooms without moving further from London.
South East double-storey extension cost per m²: £2,160 - £3,250 (standard) | £3,200 - £4,800+ (high-end)
Total project cost for a 40 m² double-storey in the South East: £100,000 - £140,000
Midlands (Birmingham, Leicester, Coventry, Nottingham)
The Midlands offers more competitive pricing, with costs typically 10-15% below the UK average. Double-storey extensions are very popular in the Midlands, particularly for semi-detached houses in suburban areas where families want to add a bedroom above an extended kitchen.
Midlands double-storey extension cost per m²: £1,530 - £2,125 (standard) | £2,400 - £3,400+ (high-end)
Total project cost for a 40 m² double-storey in the Midlands: £80,000 - £115,000
North (Manchester, Leeds, Liverpool, Newcastle)
Northern regions offer some of the best value for double-storey extensions, with costs 15-20% below the UK average. Labour is more affordable, and competition among builders keeps prices keen. Victorian terraced houses in Manchester and Leeds are particularly well-suited to double-storey rear extensions.
North double-storey extension cost per m²: £1,440 - £2,000 (standard) | £2,200 - £3,200+ (high-end)
Total project cost for a 40 m² double-storey in the North: £75,000 - £110,000
Scotland (Edinburgh, Glasgow)
Scotland generally falls 10-20% below the UK average for construction costs, though Edinburgh's historic properties and conservation area restrictions can push prices higher. The Party Wall etc. Act 1996 does not apply in Scotland.
Scotland double-storey extension cost per m²: £1,440 - £2,100 (standard) | £2,200 - £3,400+ (high-end)
Total project cost for a 40 m² double-storey in Scotland: £75,000 - £115,000
For a precise cost estimate based on your postcode, use Suddeco's free render tool. Our AI analyses local planning precedents and regional cost data to give you an accurate, location-specific estimate.
What Is a Double-Storey Extension?
A double-storey extension (also called a two-storey extension) extends both the ground floor and the first floor of your home simultaneously. Unlike a single-storey extension, which only adds space at ground level, a double-storey extension gives you two full floors of new living space for a single foundation and roof.
Common Layouts for Double-Storey Extensions
| Floor | Typical Use | Popular Features |
|---|---|---|
| Ground floor | Kitchen-diner, living room, utility, or home office | Bi-fold doors, open-plan layout, underfloor heating, kitchen island |
| First floor | Extra bedroom, master suite with en-suite, family bathroom, or home office | Ensuite bathroom, built-in wardrobes, Juliet balcony, large windows |
Why Homeowners Choose a Double-Storey Extension
- Cost efficiency: The cost per m² is lower than a single-storey extension because the foundation and roof are shared across two floors. Adding the second floor typically costs only 40-50% more than the ground floor alone — far cheaper than building two separate single-storey extensions.
- No garden loss: Unlike a large single-storey extension, a double-storey extension uses the same ground footprint, preserving more of your garden.
- Property value: According to the HomeOwners Alliance, a double-storey extension can add 20-25% to your property's value — more than a single-storey extension — because it adds bedrooms, which are highly valued by buyers.
- Avoiding moving costs: For families who need an extra bedroom but love their location, a double-storey extension is often cheaper than moving to a larger house when stamp duty, estate agent fees, and removal costs are factored in.
- Planning advantages: Double-storey extensions at the rear often fall within permitted development rights, making them easier to approve than complex single-storey designs.
Common Property Types for Double-Storey Extensions
- Semi-detached houses (especially in suburban areas)
- Detached houses (most flexibility in design and size)
- Victorian and Edwardian terraced houses (rear double-storey extensions)
- 1930s-1950s semi-detached houses (the most common type for double-storey extensions)
Double-Storey vs Single-Storey Extension
| Factor | Single-Storey Extension | Double-Storey Extension |
|---|---|---|
| Cost per m² | £1,800 - £2,800 | £1,800 - £2,500 |
| Total cost for 40 m² | £72,000 - £112,000 | £90,000 - £125,000 (inc. fees) |
| Garden space lost | More (full footprint) | Less (same ground footprint) |
| Value added | 10-15% | 20-25% |
| Best for | Open-plan kitchen-diners, bungalow extensions | Extra bedrooms, family bathrooms, maximum value |
| Planning complexity | Often simpler | Slightly more complex but often still permitted development |
For more detail on single-storey extension costs, read our single-storey extension cost guide.
What Affects the Cost?
The final price of your double-storey extension depends on several interconnected factors. Understanding these helps you budget accurately and avoid surprises.
1. Size and Footprint
The ground floor footprint determines the total floor area, as the first floor matches the same dimensions. A larger footprint costs more in absolute terms but the cost per m² often decreases slightly as the size increases because fixed costs (foundations, roof, scaffolding) are spread over a larger area. However, very large extensions may require planning applications even if they fall within permitted development limits, adding cost and time.
2. Materials and Finish Quality
Your choice of materials has a significant impact on the final cost:
| Element | Budget Option | Standard Option | Premium Option |
|---|---|---|---|
| Ground floor fit-out | Vinyl flooring, basic kitchen | Engineered wood, mid-range kitchen | Natural stone, bespoke kitchen |
| First floor fit-out | Carpet, basic bathroom | Engineered wood, quality bathroom | Luxury tiles, designer bathroom |
| Windows | Standard UPVC | Aluminium or composite | Premium timber or aluminium-clad timber |
| Roof | Standard pitched roof, concrete tiles | Pitched roof with Velux windows | Complex roof design, dormer windows |
| Staircase | Straight staircase, basic materials | Straight or winder staircase, quality materials | Feature staircase, glass balustrade |
| Heating | Standard radiators | Underfloor heating (ground floor) | Smart heating, underfloor heating both floors |
3. Foundation Type
Ground conditions determine the foundation type required: - Standard strip foundations: Most cost-effective for good ground conditions (£100-£150/m² of foundation) - Reinforced strip foundations: Required for poorer soils or deeper excavations - Piled foundations: Necessary for very poor ground or high water tables (£2,000-£5,000 additional) - Party wall foundations: If building within 3 metres of a neighbour's wall, specific foundation designs may be required under the Party Wall Act
4. Structural Complexity
A double-storey extension requires careful structural engineering to ensure the new first floor is properly supported and the existing house structure is not compromised. Key structural elements include: - Steel beams (RSJs): To support the new first floor and connect it to the existing structure (£3,000 - £6,000) - Foundations: Must be deeper and stronger than for a single-storey extension to support the additional load - Roof structure: Must tie into the existing roof, often requiring partial re-roofing of the original property (£2,000 - £5,000) - Chimney modifications: If the extension affects an existing chimney, modifications may be needed (£1,000 - £3,000)
5. Location and Access
As detailed above, location significantly affects labour costs. Site access also matters — properties with narrow side passages, no rear access, or parking restrictions can increase costs by 10-20%. Double-storey extensions require scaffolding for the first floor construction, which adds £1,000 - £3,000 to the project cost compared to a single-storey extension.
6. Ground Floor Fit-Out
The ground floor of a double-storey extension is typically a kitchen-diner or living space. As with any extension involving a kitchen, the fit-out cost can range from £8,000 to £60,000+ depending on your choices. For a mid-range kitchen in a 20 m² ground floor space, budget £15,000 to £25,000.
7. First Floor Fit-Out
The first floor is typically a bedroom or bathroom. Costs include: - Bedroom: £3,000 - £8,000 (flooring, decoration, built-in wardrobes, lighting) - Bathroom: £5,000 - £15,000 (suite, tiling, plumbing, underfloor heating) - En-suite: £4,000 - £12,000 (smaller suite, premium finishes)
8. Glazing and External Features
- Standard windows: £300 - £800 per window (UPVC)
- Aluminium windows: £600 - £1,500 per window
- French doors: £1,000 - £2,500
- Bi-fold doors (ground floor): £3,000 - £6,000
- Velux roof windows: £500 - £1,200 each
- Dormer windows: £3,000 - £8,000 each
9. Planning and Regulatory Constraints
Some properties face additional planning constraints: - Conservation areas: May require planning permission where permitted development would normally apply; additional design constraints - Listed buildings: Full planning permission always required; specialist conservation architects and materials needed - Article 4 directions: Some councils have removed permitted development rights for specific areas - Designated land (AONB, National Parks): Stricter planning controls apply - Right to light issues: If your extension affects a neighbour's light, they may have a legal claim
Hidden Costs to Budget For
Many homeowners underestimate the total project cost by focusing only on construction. The following costs can add £15,000 to £30,000 to your budget:
| Cost Item | Typical Price Range | Notes |
|---|---|---|
| Planning permission (householder) | £206 | Required if not fully covered by permitted development |
| Planning permission (full) | £462 | For listed buildings or complex applications |
| Building regulations | £200 - £1,000 | Mandatory for all extensions |
| Structural engineer | £500 - £2,000 | Calculations for steel beams, foundations, and floor joists |
| Party wall surveyor | £700 - £2,000 | Required if building within 3m of a neighbour's wall |
| Architect fees | £3,750 - £7,500 | 5-10% of construction cost for a typical project |
| Contingency | 10-15% of build cost | Essential for unexpected issues |
| Scaffolding | £1,000 - £3,000 | Required for first floor construction |
| Building control inspections | £200 - £500 | Inspections at foundation, steelwork, and completion stages |
| Utility connections | £500 - £2,000 | Moving gas, electric, water, and drainage connections |
| Kitchen appliances | £2,000 - £10,000 | Not always included in builder quotes |
| Bathroom suite and fixtures | £2,000 - £8,000 | Varies significantly by quality |
| Garden reinstatement | £1,000 - £5,000 | Patio, turf, fencing after construction |
| Decoration and finishes | £2,000 - £8,000 | Painting, tiling, flooring, lighting, fixtures (both floors) |
Total hidden costs for a £100,000 build: £15,000 - £32,000
Always add a 10-15% contingency to your construction budget. Asbestos removal, unexpected drainage issues, poor ground conditions, or discovering that existing walls are not structurally sound can all increase costs significantly once work begins.
How to Save Money on a Double-Storey Extension
A double-storey extension is a significant investment, but there are several proven strategies to reduce costs without sacrificing quality:
1. Use Permitted Development Rights Where Possible
Under the UK's permitted development rules, you can build a double-storey rear extension up to 3 metres from the rear wall of an attached house (4 metres for detached), up to the height of the existing roof. The total extension cannot exceed 50% of the original garden area. Using permitted development avoids the £206 planning fee and several weeks of waiting time.
However, the rules are complex, and double-storey extensions that extend to the side or that are particularly large may still require planning permission. Use Suddeco's free render tool to see whether your specific project would require planning permission based on your address and local precedent data.
2. Choose Standard Materials Over Premium
The difference between budget and premium materials can be £20,000 or more across both floors. Quality mid-range options deliver excellent results at a fraction of the cost: - Kitchen: Howdens or IKEA with quality worktops (£12,000 - £18,000) vs. designer brands (£30,000+) - Bathroom: Quality mid-range suites (£3,000 - £6,000) vs. premium designer suites (£8,000 - £15,000) - Windows: UPVC or standard aluminium (£300 - £800 per window) vs. premium timber (£1,000 - £2,000 per window) - Flooring: Large-format porcelain or engineered wood (£40 - £80/m²) vs. natural stone or premium hardwood (£100 - £200/m²) - Roof: Standard concrete tiles (£40 - £60/m²) vs. premium slate (£80 - £150/m²)
3. Get Competitive Quotes from Multiple Builders
The FMB recommends obtaining at least three detailed quotes from reputable builders. Prices can vary by 20-30% between builders for the same specification. Ask for itemised quotes so you can compare like-for-like and identify any builder who is cutting corners or overcharging on specific elements.
4. Avoid Moving the Kitchen or Bathroom Plumbing
Keeping your kitchen and bathroom layouts as close as possible to the existing plumbing and drainage can save £2,000 to £5,000 in relocation costs. Moving a soil stack, relocating drains, or rewiring the first floor adds significant expense and complexity.
5. Do the Decoration Yourself
Builders' quotes for painting, tiling, and finishing both floors can be high. If you are competent at DIY, consider doing the decoration yourself after the builder has completed the structural work. This can save £3,000 to £6,000 across two floors.
6. Use Suddeco's AI Design Platform
Traditional architects charge 5-10% of construction cost for design services, and the process can take 6-12 weeks. Suddeco's AI platform generates architect-quality concept drawings, 3D renders, and full planning packages in 48 hours for a fraction of the cost. Our Concept Package starts at just £495, and our full Design+Planning service is £2,495 — saving you thousands on design fees and weeks of waiting time.
7. Build in the Off-Season
Builders are often quieter in January to March and October to November. Booking your project during these periods can sometimes secure a 5-10% discount. Avoid peak spring and summer when demand is highest.
8. Consider a Simple Roof Design
A complex roof with multiple valleys, dormers, or a bespoke design can add £5,000 to £15,000 to the project. A simple pitched roof that mirrors the existing roofline is the most cost-effective option and is often more likely to gain planning approval.
Double-Storey vs Single-Storey Extension
If you are deciding between extension types, here is a direct comparison of costs, benefits, and suitability:
| Factor | Single-Storey Extension | Double-Storey Extension |
|---|---|---|
| Cost per m² | £1,800 - £2,800 | £1,800 - £2,500 |
| Total cost for 40 m² | £72,000 - £112,000 | £72,000 - £100,000 |
| Total cost for 40 m² (inc. fees) | £85,000 - £130,000 | £90,000 - £125,000 |
| Garden space lost | Full footprint (e.g., 40 m²) | Same ground footprint (e.g., 20 m²) |
| Value added | 10-15% | 20-25% |
| Best for | Open-plan kitchen-diners, bungalows | Extra bedrooms, maximum space, value |
| Planning complexity | Usually simpler | Often permitted development (rear only) |
| Disruption | Medium (8-12 weeks) | High (12-20 weeks) |
| Scaffolding cost | Lower | Higher (first floor requires scaffolding) |
A double-storey extension is the best choice if you need extra bedrooms or want maximum value for your investment. The cost per m² is lower, you lose less garden space, and the property value increase is significantly higher. However, if you only need ground floor space and want to minimise disruption and upfront cost, a single-storey extension may be more appropriate.
For more detail on single-storey extension costs, read our single-storey extension cost guide. For general extension costs, see our home extension cost guide.
Planning Permission and Building Regulations
Permitted Development for Double-Storey Extensions
Double-storey extensions at the rear of a property often qualify for permitted development rights, provided they meet the following criteria: - Rear extension: Maximum 3 metres beyond the rear wall for terraced/semi-detached houses (4 metres for detached) - Height: Must not exceed the height of the existing roof - Eaves height: Maximum 3 metres - Total footprint: The extension and any other extensions cannot exceed 50% of the total area of land around the original house - Side elevation: No windows at first floor level on side elevations within 15 metres of a boundary - Materials: Must be similar in appearance to the existing house - Article 4: Some areas have had permitted development rights removed
If your double-storey extension extends to the side, or if your property is in a conservation area, listed, or subject to an Article 4 direction, you will need to submit a householder planning application (£206 fee). The application typically takes 8-13 weeks for a decision.
Building Regulations
All double-storey extensions must comply with Building Regulations, regardless of whether planning permission is required. Key areas of compliance include: - Structural stability: Foundations, steel beams, floor joists, and roof structure must support the additional load - Thermal insulation: Walls, floors, and roofs must meet current U-value requirements (more stringent than for single-storey extensions) - Fire safety: Escape routes, fire doors, smoke alarms, and fire-resistant construction between floors - Ventilation: Extractor fans, trickle vents, and whole-house ventilation requirements - Drainage: Surface water and foul water drainage must be properly designed - Electrical safety: Part P compliance for all new electrical work - Staircase design: Must meet Building Regulations for rise, going, headroom, and handrails
Building Control fees for a double-storey extension range from £300 to £1,200 depending on the council and the size of the project.
For help navigating planning permission and building regulations, Suddeco's Design+Planning package includes full planning application preparation, building regulations drawings, and structural engineering calculations — all for £2,495.
Do You Need an Architect?
For a double-storey extension, professional design input is strongly recommended because of the structural complexity involved — particularly the connection between the new structure and the existing house, and the integration of the new first floor with the existing roof.
Options for Design Services:
| Option | Cost | Pros | Cons |
|---|---|---|---|
| Traditional architect | £3,750 - £7,500 (5-10% of build) | Full design service, site visits, planning expertise | Expensive, slow (6-12 weeks), may prioritise design over buildability |
| Architectural technician | £2,000 - £4,000 | Cheaper, faster, more focused on buildability | Less design flair, limited planning support |
| Suddeco AI platform | £495 - £2,495 | 48-hour delivery, planning-ready, cost-optimised, precedent-based | No site visits (uses satellite data and address intelligence) |
What Suddeco's AI Design Service Includes:
- Free render: Instant 3D visualisation based on your address and nearby approved plans
- Concept Package (£495/£995): 3 renders, full floor plans, elevation drawings, 1 revision, 48-hour delivery
- Design+Planning (£2,495): Full planning application, building regulations drawings, structural engineer coordination, unlimited revisions, planning support
Our platform analyses thousands of approved planning applications in your area to design extensions that are highly likely to be approved. We do not design in a vacuum — we design based on what your local council has already approved, dramatically increasing your chances of first-time planning success.
See our full pricing or get a free render now.
Get Your Free AI Render
Before you commit tens of thousands of pounds to a double-storey extension, see exactly what is possible for your property. Suddeco's AI platform:
- Reads your address and analyses your property's footprint, orientation, and existing roofline
- Searches nearby approved planning applications to understand what your local council has already approved
- Generates an instant 3D render showing your double-storey extension in situ
- Provides a cost estimate based on your location and project size
- Delivers planning feasibility — whether your project is likely to require planning permission or qualifies for permitted development
Get your free AI render today →
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Frequently Asked Questions
What is the cheapest a double-storey extension can cost?
The cheapest viable double-storey extension in the UK costs approximately £54,000 to £65,000 for construction, with total project costs around £70,000 to £85,000 including fees and contingency. This assumes a smaller size (15 m² ground floor / 30 m² total), basic materials, and a location outside London. Anything below £50,000 for construction risks significant quality compromises. Checkatrade and FMB data consistently show that the lowest quotes are often followed by substantial cost overruns.
How long does a double-storey extension take to build?
A typical double-storey extension takes 12 to 20 weeks to build, depending on size and complexity. The project timeline usually breaks down as follows: - Weeks 1-3: Foundations, groundworks, and drainage - Weeks 4-6: Ground floor walls, structural steelwork, and first floor joists - Weeks 7-9: First floor walls, roof structure, and tiling - Weeks 10-13: Windows, doors, and external finishes - Weeks 14-17: First fix (electrics, plumbing, heating on both floors) - Weeks 18-20: Second fix (kitchen, bathrooms, flooring, decoration on both floors)
Design and planning add 6-16 weeks before construction begins. Suddeco's AI platform reduces the design phase to 48 hours, and our Design+Planning package prepares planning applications in 2-3 weeks rather than the typical 6-12 weeks.
Does a double-storey extension need planning permission?
Not always. Double-storey extensions at the rear of a property often qualify for permitted development if they meet the rules: maximum 3 metres beyond the rear wall for attached houses (4 metres for detached), not exceeding the height of the existing roof, and not covering more than 50% of the garden area. However, if your property is in a conservation area, listed, subject to an Article 4 direction, or if the extension extends to the side, you will need planning permission. Use Suddeco's free render tool to check your specific property's planning status.
How much value does a double-storey extension add?
A well-designed double-storey extension can add 20-25% to your property's value, according to the HomeOwners Alliance. This is higher than a single-storey extension because it adds bedrooms — the most valuable type of room for buyers. In London and the South East, where extra bedrooms command a significant premium, the value increase can be even higher. For a £400,000 property, a £100,000 double-storey extension can increase the value to £480,000-£500,000 — a strong return on investment.
Is a double-storey extension cheaper than two single-storey extensions?
Yes, significantly. A double-storey extension shares the same foundation, roof, and scaffolding across two floors, making it far more cost-efficient than building two separate single-storey extensions at different times. The second floor typically adds only 40-50% more cost than the ground floor alone. Building two separate single-storey extensions would cost approximately 60-80% more in total because you would pay for foundations, roof, and scaffolding twice.
What is the Party Wall Act and do I need a party wall agreement?
The Party Wall etc. Act 1996 applies if you are building within 3 metres of a neighbouring property's wall or excavating to a depth that extends below the neighbour's foundations. For a double-storey extension, a party wall agreement is almost always required because you are building up to or near the boundary. The cost is £700-£2,000 for a party wall surveyor, and you must serve notice to your neighbours at least 2 months before work begins. In Scotland, different rules apply.
How much does a staircase cost in a double-storey extension?
A new staircase connecting the existing first floor to the new extension typically costs £2,000 to £6,000, depending on design and materials: - Straight staircase, basic materials: £2,000 - £3,500 - Winder staircase (to save space): £3,500 - £5,000 - Feature staircase with glass balustrade: £5,000 - £10,000+
The staircase must also meet Building Regulations for rise, going, headroom (minimum 2 metres), and handrail height.
What are the main causes of cost overruns on double-storey extensions?
The most common causes of budget overruns are: 1. Poor ground conditions requiring deeper foundations or piled foundations (£2,000-£5,000 extra) 2. Asbestos discovery in old soffits, roof tiles, or Artex ceilings (£1,000-£5,000 for removal) 3. Unexpected roof integration issues (£2,000-£8,000) 4. Scope creep — adding features during construction (£2,000-£10,000+) 5. Utility relocation (gas meter, electricity supply, drainage) (£500-£2,000) 6. Party wall disputes requiring additional surveyor fees (£500-£1,500) 7. Scaffolding extensions if the project runs over schedule (£500-£1,500 per week)
The best protection is a 10-15% contingency fund and a thorough pre-construction survey.
Is a double-storey extension cheaper than moving house?
In most cases, yes. Moving house from a £400,000 property to a £500,000 property incurs stamp duty (£12,500), estate agent fees (£6,000), legal fees (£2,000), and removal costs (£1,000) — approximately £21,500 in total. A £100,000 double-storey extension that adds £80,000-£100,000 in value is often more cost-effective than moving, especially in areas where larger properties are scarce or in catchment areas for good schools. You also avoid the stress and disruption of moving.
What is the best time of year to build a double-storey extension?
The best time to start construction is spring (March-May) or early autumn (September-October). These periods offer: - Good weather for foundations and external work - Builders are more available than in peak summer - You can have the project weather-tight before winter, avoiding delays - The second fix can be completed in comfortable working temperatures
Avoid starting in December or January if possible, as cold weather and short days slow progress. Scaffolding costs also increase in winter if high winds or snow make it unsafe to work at height.
Can I get a mortgage or loan to fund a double-storey extension?
Yes, several options exist: - Further advance on your mortgage: Extend your existing mortgage at current rates. Often the cheapest option. - Remortgage: Switch to a new lender with a larger loan amount. Best if your current deal is ending. - Secured home improvement loan: A second charge on your property. Rates are higher than mortgages but lower than unsecured loans. - Unsecured personal loan: £25,000-£50,000 maximum. Higher rates (6-15% APR) but no property security required. - Bridging loan: Short-term option if you need funds quickly. Expensive (0.5-1.5% per month) but fast to arrange.
Most homeowners use a combination of savings and a further advance or remortgage. Speak to a mortgage broker to find the best option for your circumstances.
How do I find a good builder for a double-storey extension?
The FMB recommends the following approach: 1. Get recommendations from friends, neighbours, or local architects who have had similar work done 2. Check credentials: FMB membership, TrustMark registration, and valid insurance (public liability and employer's liability) 3. View previous work: Ask to see completed double-storey extensions and speak to past clients 4. Get 3 detailed quotes: Itemised quotes so you can compare like-for-like 5. Check references: Call at least two previous clients and ask about budget adherence, quality, and communication 6. Avoid cash-in-hand deals: Always use a formal contract with payment terms tied to milestones
Alternatively, Suddeco's Design-to-Build service matches you with vetted builders from our network, provides project management, milestone payment protection, and a 12-month warranty — removing the risk of choosing the wrong builder.
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