Extension Quote: Get a Free Estimate and an Itemised Price in 2026
Last updated: 4 October 2026
Want a real number, not a range? Put in your address and we will confirm your project on a free survey. Book my free survey
A quote is the point where an extension stops being a budget and becomes a decision. This page is about getting one: start with an instant online estimate, then confirm it on a free survey and receive an itemised quote against your own drawings.
We are a London design-and-build company, so the number you are quoted is the number we build to — not a figure handed to a third party. Design and build means drawings, planning, structure and construction sit with one team.
Extension cost estimator
Indicative only. Your survey gives the real number.
How to Get Your Extension Quote
Three steps, and only the last one costs you any time.
1. Get an instant estimate
Use the estimator above for an indicative figure. It takes under a minute and needs no contact details. Treat it as a starting position rather than a price — it cannot know about the soil, the party wall or the access at your address.
2. Book a free survey
We visit, measure, and check the things a calculator cannot: drainage runs, structural openings, head height, access for plant, and what your local authority has approved nearby. There is no charge and no obligation. Book a free survey.
3. Receive your itemised quote
You get a written quote broken down by work package — groundworks, structure, envelope, services, finishes and professional fees — so you can see what each part of the build costs and where the contingency sits.
What Your Extension Quote Will Include
A figure on its own is not a quote. Yours should itemise:
| Work package | What it covers |
|---|---|
| Groundworks | Excavation, foundations, drainage and any underpinning |
| Structure | Steel, lintels, floor and roof structure, structural engineering |
| Envelope | Walls, insulation, glazing, roofing and weatherproofing |
| Services | Electrical, plumbing and heating, ventilation |
| Finishes | Plastering, flooring, decoration |
| Fees and approvals | Planning, building regulations, party wall |
| Contingency | A stated allowance, visible rather than buried in the rates |
Why Extension Quotes Vary So Much
Two quotes for the same extension can differ by half. The reasons are knowable in advance, and all of them are checked on the survey:
- Ground conditions. Clay, trees nearby and a high water table all change the foundation design. This is the single most common cause of a quote moving after it is issued.
- Structure. A wide opening needs a steel beam sized by an engineer. Structural work is priced by weight and span, not by floor area.
- Access. If a digger cannot reach the rear, everything is carried by hand — and that is labour.
- Services. Moving a consumer unit, a boiler or a soil stack is often the difference between a simple extension and a complicated one.
- Specification. Glazing, kitchens and floor finishes swing the total more than the build itself.
- Approvals. Planning, building control and party wall each carry fees and time.
For a full breakdown of what drives the per-square-metre figure, see the extension cost guide. If you would rather price the trades individually, our construction services pages set out each one.
Quotes for Lofts, Basements and Renovations
The same three steps apply to every project type we build. The survey changes, the process does not.
Dormer, hip-to-gable and mansard. Head height and structure decided on site.
Underpinning, waterproofing and light wells — quoted as a package.
Room by room or the whole property, phased if you need to live in.
Frequently Asked Questions
How do I get an extension quote?
Start with the instant estimate on this page, then book a free survey. We measure your property, check the structure, access and local planning context, and send you a written itemised quote. You can also start directly with the survey if you already have drawings.
Is an extension quote free?
Yes. The survey and the itemised quote are free and there is no obligation to proceed.
Is an estimate the same as a quote?
No, and the difference matters. An estimate is an indicative figure based on typical costs — useful for planning a budget, before anything is known about your property. A quote is a fixed, itemised price for defined work after a survey. Treat the estimator on this page as an estimate and the written quote as the number you can hold us to.
How long does it take to receive an extension quote?
The instant estimate is immediate. The survey is usually booked within a few days, and a written itemised quote follows once we have measured and checked the site.
Can I get an extension quote online?
You can get an online estimate immediately, and you can submit your project online. The quote itself follows a survey, because the things that move an extension price — ground conditions, structure, access and services — cannot be read from a web form.
What information do you need to quote my extension?
Your address, roughly what you want to build, and any drawings if you have them. Drawings are not required to start — we can be involved from the first sketch through architectural drawings and planning.
Do you quote for loft conversions, basements and renovations as well as extensions?
Yes. We quote the same way for loft conversions, basement conversions, whole-house renovations, kitchens, bathrooms and garage conversions.
Why is my quote higher than my estimate?
Usually one of four things: ground conditions needing deeper foundations, a structural opening needing a larger steel, restricted access adding labour, or services that have to be moved. All four are identified on the survey, before the quote is written — which is the point of doing it in that order.
Do you cover my area?
We work across London. See the boroughs we cover, or check what has been approved near you.
Get Your Extension Quote
Put in your address and we will confirm your project on a free survey, then send an itemised quote against your own drawings.