House Extension Cost in London: 2026 Prices by Type and Size

Last updated: 3 October 2026

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A house extension in Greater London costs roughly £2,000–£4,200 per m² including VAT, depending on the type of extension and the specification. That is the span of the per-square-metre figures we already publish across our London extension guides, and it is deliberately wide: a straightforward kitchen extension and a high-specification rear extension on a constrained site are different products, even when the floor area is identical.

What follows is the London picture by extension type, the fees that sit on top of the build cost, and the items that move a London quote more than the specification does. Every figure is one we already publish in a Suddeco Homes guide, so you can trace each number to its source. They are indicative 2026 estimates including VAT at 20%, not quotations.

House extension cost in London at a glance

Start with the type of extension, not with a headline total. The table below sets out the published London rates for each type, with the total for a typical London size.

Two cautions apply to every London extension cost table, including this one. First, the per-square-metre rate is not a price list: it is the middle of a range that a foundation design, a scaffolding licence or a specification decision can move. Second, the total column is a budget, not a quote. Our published worked example for a 20 m² London extension — construction at £2,800 per m² plus structural engineering, building control, party wall, the planning fee and design — comes to roughly £68,000–£71,000 all-in, and the model behind it is set out line by line in the house extension cost calculator.

What the London premium is made of

London extension rates sit roughly 30–50% above the national average. The premium is not one surcharge; it is a stack of smaller costs, and most of them appear before a single brick is laid.

• Labour. London trades charge materially more per day, and an extension is labour-heavy: groundworks, framing, roofing, plastering and finishing are hand time rather than material cost. Our published London day rates for the finishing trades are £250–£350 for a fitter against £180–£250 outside the capital, and £200–£350 for a plumber.
• Access. Narrow side passages and no rear access mean materials are carried through the house and waste leaves by van rather than skip. Scaffolding and a parking suspension are separate line items on most London streets.
• Party wall. Almost every London extension either builds off a shared wall or excavates near a neighbour's foundation, which brings the Party Wall etc. Act 1996 into play. The published London figures are £1,200–£2,500 for an agreed surveyor, or £2,800–£6,000 where two surveyors are appointed.
• Period structure. Victorian and Edwardian housing stock brings solid walls, suspended timber floors, drainage that has been altered several times, and lead supply pipework. On pre-1980 property, assume an asbestos survey at £200–£400 and removal at £200–£1,500 or more.
• Conservation areas and Article 4 directions. Large parts of inner London have permitted development rights restricted or removed, which means a full application, a heritage statement where relevant, and a longer programme before work can start.
• Ground conditions. Where the soil, a high water table or nearby trees demand deeper foundations, our single-storey extension guide puts the addition at £2,000–£10,000.

What sits on top of the build cost

The build cost is not the project cost. The published London figures for the fee stack are below. The planning fee and building control fees are not subject to VAT; the rest are quoted inclusive.

Add the rows that apply to your scheme, then add contingency. The most common budgeting error we see is comparing a construction-only price from one builder with an all-in price from another; the second figure looks expensive because it is complete.

How size and type change the number

Every extension carries costs that do not scale with area. Design, the planning fee, building control, the party wall award, scaffolding, the skip and the site setup are much the same on a 12 m² scheme as on a 30 m² one. Spread them over fewer square metres and the rate rises. That is why the smallest side returns sit at the top of the per-square-metre range even though the total is the lowest in our tables: the published side return rates are £2,000–£3,000 per m², but the typical 8 m² job is £18,000–£27,000 all-in.

Two storeys share their foundations and roof. Our published London rates put a standard double-storey extension at £2,340–£3,750 per m² against £2,340–£4,200 per m² for a general single-storey extension. The saving is real but it is per square metre, not in total: a 40 m² double-storey extension costs considerably more than a 20 m² single-storey one because you are building twice the floor area.

Specification moves the all-in total further than most homeowners expect. For a 20 m² London rear extension, the published all-in bands run from £41,000–£50,000 for a budget build to £61,000–£74,000 at premium specification — the same footprint and the same planning process, with a different finish.

Fit-out is usually priced separately, and it is not small. A kitchen extension carries the construction cost plus £10,000–£25,000 for mid-range units and appliances. Glazing is the other line that surprises: bi-fold or sliding doors are published at £1,500–£4,000 per linear metre installed, and a structural glass roof at £3,000–£8,000.

Planning, party wall and building regulations in London

A single-storey rear extension can often be built under permitted development, in which case you avoid the £206 householder fee and the weeks a decision takes. Conservation areas, listed buildings and Article 4 directions remove or restrict those rights, and London has a high concentration of all three, so the address matters as much as the drawings. Our guides to what you can build at my address and rear extension cost in London cover the permitted development limits in detail.

Building regulations approval is separate from planning and is required for every extension, whether or not planning permission is needed. It covers structure, thermal performance, drainage, ventilation, electrics and fire. Party wall notices are the most common cause of a delayed start on London schemes, because the notice period runs before work can begin — serve them as soon as the drawings are fixed.

What moves a London extension quote

• Foundations. Deeper foundations or piling add £2,000–£10,000 on the published figures, and nothing visible improves.
• Drainage and services. Moving drains, a build-over agreement, or relocating a gas or electricity supply is published at £1,000–£5,000.
• Access. A typical access restriction adds £1,000–£3,000 to the published model before scaffolding and parking are counted.
• Structural alterations. Removing an internal wall to open up the existing house adds £2,000–£5,000, plus a steel beam at £1,500–£3,000 and the engineer's calculations.
• Glazing and roof. Bi-folds, structural glass and large roof lanterns (£1,500–£5,000) are specification choices with structural consequences.
• VAT. Everything except the statutory fees carries 20%. Establish whether each quote includes it, because a 20% difference on a £70,000 job is £14,000.

Where London extension budgets go wrong

• Comparing a construction-only price with an all-in price, and choosing the first because the number is smaller.
• Buying the kitchen or the glazing before the layout is fixed, then discovering the run does not fit.
• Budgeting for the build but not the fee stack: design, engineering, building control, party wall, survey and contingency.
• Running no contingency on period property. The three most common findings on a London strip-out are a rotten subfloor, damp, and asbestos — all invisible until work starts.
• Accepting a single lump-sum quote. Ask every builder for a written schedule of works broken down by element so the quotes can actually be compared.

Getting a comparable London price

London extension costs converge only when the design is settled. Two builders pricing the same drawings, the same schedule of works and the same specification will produce comparable numbers; two builders pricing “an extension” will not.

Our free concept service checks your address and what has been approved nearby, then produces concept drawings with an indicative London price. If you want measured drawings to put in front of builders, the Concept package is £995. Design + Planning, which covers the planning application, building regulations drawings and structural engineering, is £2,495. Design to Build, where we build it at a fixed quote, starts at £4,500. We work across Greater London only.

We design it, we build it. If you want to know what your address can take before you commit to a budget, get in touch.

Frequently asked questions

How much does a house extension cost in London?

Between roughly £2,000 and £4,200 per m² including VAT, depending on the type of extension and the specification. A 20 m² general single-storey extension is published at £47,000–£84,000, and our published worked example for a 20 m² London extension comes to about £68,000–£71,000 all-in once the fee stack and contingency are included. These are indicative 2026 estimates, not quotations.

What is the cost per m² for an extension in London?

Published London rates run from £2,000–£3,000 per m² for a side return or a kitchen extension, through £2,300–£3,500 per m² for a rear extension depending on whether you are in zones 1–3 or outer London, to £2,340–£4,200 per m² for a general single-storey extension and £2,340–£3,750 per m² for a standard double-storey. All figures include VAT.

Why are house extensions more expensive in London?

Higher day rates for trades, restricted access with scaffolding and parking licences, near-universal party wall involvement at £1,200–£2,500 for an agreed surveyor, period housing stock with solid walls and altered drainage, and conservation areas or Article 4 directions that remove permitted development rights. The premium is a stack of smaller costs rather than one London surcharge.

Does a house extension need planning permission in London?

A single-storey rear extension can often be built under permitted development, which avoids the £206 householder application fee. Conservation areas, listed buildings and Article 4 directions remove or restrict those rights, and inner London has a high concentration of all three. Where permission is needed, a householder application is £206 and a full application £462, neither subject to VAT.

What fees sit on top of the build cost?

Design at 5–10% of build cost, which is £3,750–£7,500 on a £75,000 project; a structural engineer at £1,100–£2,500; building regulations at £600–£1,000 through the borough or £900–£1,200 through a private inspector; party wall at £1,200–£2,500 for an agreed surveyor; a site survey at £400–£1,000; and a contingency of 10–15% of build cost.

How much does a 20 m² extension cost in London?

Our published worked example uses construction at £2,800 per m² for 20 m², which is £56,000, plus structural engineering at £1,100–£2,500, building regulations at £600–£1,000, party wall at £1,500–£2,500, the £206 planning fee, design at £995–£2,495 and contingency, giving roughly £68,000–£71,000 all-in.

Is a double-storey extension cheaper per square metre than a single-storey one?

Yes, on our published London rates: £2,340–£3,750 per m² for a standard double-storey against £2,340–£4,200 per m² for a general single-storey extension, because foundations and the roof are shared across two floors. The total is still much higher because you are building twice the floor area: a 40 m² London double-storey is published at £105,000–£150,000.

How do I get a London extension price I can compare?

Fix the design first. Two builders pricing the same drawings, the same schedule of works and the same specification will produce comparable numbers; two builders pricing “an extension” will not. Ask for a written schedule of works broken down by element, confirm whether VAT is included, and compare the all-in figure rather than the construction figure alone.