What does an extension cost in Swindon?
The honest answer is a range — and any builder who gives you a single number before visiting is guessing. Here are our published 2026 guide prices, what actually moves the number, and how to budget without nasty surprises.
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| Single-storey | £2,500–£3,200/m² |
|---|---|
| Double-storey | £2,200–£2,800/m² |
| Loft conversions | £1,800–£2,600/m² |
| Garage conversions | £1,400–£2,000/m² |
| Your project | Fixed written quotation |
What those rates mean for a real project
Guide rates only mean something when you multiply them out. A typical 4m × 5m single-storey kitchen extension is 20 m² — at £2,500–£3,200 per m² that’s roughly £50,000–£64,000 built and certified. A 6m × 4m double-storey addition (48 m² over two floors) at £2,200–£2,800 lands around £106,000–£134,000. Converting a standard single garage of about 13 m² is usually £18,000–£26,000.
These are indicative ranges for budgeting, not quotes — your ground conditions, design and specification move the number, which is why every SwiftBrick project is priced properly against your drawings after a site visit. But they’re honest ranges: if a figure you’ve been given elsewhere is dramatically below them, ask very carefully what’s been left out.
What actually moves the price
Size — but less than you’d think
Bigger costs more, but the rate per square metre falls as size grows: the expensive parts of any extension are the ones every project needs once — groundworks, steels, roof, connections into the existing house. That’s also why double-storey is better value per metre: one set of foundations and one roof serve two floors.
Glazing and openings
Bifolds, sliders and rooflights transform a space and carry real cost — both the units and the structural openings behind them. A wall of glass can move a project by thousands; we’ll tell you where it’s worth it and where a cheaper window does the same job.
Structure and ground
Removing the back wall of the house means engineered steel. Poor ground, nearby trees or drains that need moving mean deeper or more complex foundations. These are the classic “surprise extras” of a vague estimate — at SwiftBrick, anything with a realistic chance of arising is flagged in the quote upfront, not discovered halfway through your build.
Kitchens, bathrooms and finishes
The same shell can carry a £8,000 kitchen or a £40,000 one. Our quotes itemise what’s included so you’re comparing like for like — and you always know which choices are yours to flex.
Fees and sign-off
Budget honestly for the professional side too: architectural drawings, structural engineer’s calculations, planning fees where needed, and Building Control. We help you brief the architect, and Building Control certification on notifiable work is part of how we finish every job — it’s the paperwork that protects your money when you sell.
Planning permission, or not
Many single-storey rear extensions in Swindon fall under permitted development rights, which saves both time and fees — and Swindon Borough Council’s householder rules allow certain rear extensions without a full application. Larger, double-storey, corner-plot and conservation-area projects need approval. Luke assesses this at the site visit and tells you straight which route your project takes before you spend anything on drawings.
How to compare quotes fairly
Ask every builder the same three questions. Is the price fixed against drawings, or an estimate? Exactly what’s included — and what happens if ground conditions surprise us? And who certifies the work? A proper written quotation answers all three. It’s also worth checking the basics that protect you: insurance, verifiable credentials, an independent review record, and a guarantee that survives the builder — ours is insurance-backed for 10 years on major projects.
Common questions
How much does a single-storey extension cost in Swindon?
As a 2026 guide, single-storey extensions in Swindon typically run £2,500–£3,200 per m². A 20 m² kitchen-diner extension therefore sits roughly in the £50,000–£64,000 range depending on design and specification, before fit-out choices like kitchens.
Is a double-storey extension better value?
Per square metre, usually yes — typically £2,200–£2,800 per m², because one roof and one set of foundations serve two floors. If you may ever want the upstairs room, it’s often cheaper to build both storeys once than to add a second later.
What’s the cheapest way to add living space?
Usually a garage conversion, at roughly £1,400–£2,000 per m² — the structure already exists. Loft conversions typically run £1,800–£2,600 per m². Which one suits you depends on the house; we’ll tell you straight at a site visit.
Why do extension quotes vary so much between builders?
Mostly because they don’t price the same job. A proper quote itemises groundworks, structure, roofing, glazing, services and finishes against your actual drawings — a vague estimate leaves the gaps to be discovered mid-build. Our quotes are fixed and written: the price we agree is the price you pay.
Does an extension add more value than it costs in Swindon?
Often, but not always — it depends on the street, the house and what you build. Extra bedrooms and open-plan kitchen-diners tend to add the most. We’ll give you an honest view at the site visit, including when we think a project isn’t worth the money.
Want a real number for your project?
Send your plans or describe the idea — Luke will visit, assess it properly, and give you a fixed written quotation with everything itemised.
Request a QuotePrefer to talk? 07460 232373 · luke@swiftbrick.co.uk