How Much Does a House Extension Cost in the UK in 2026?
- Real Life Architecture
- Jul 2
- 4 min read
How much does a house extension cost? This is the question I have been asked more than any other in my career.
I am an architect and I have been running my own practice in Edinburgh since 2009. In that time I have designed and built dozens of house extensions across Edinburgh, the Lothians and Fife. The figures below are not from a cost calculator or a lender's survey. They are construction costs from my own built projects, updated for 2026, so you can see exactly what your money buys.
One thing before we start. Every price here is quoted plus VAT, because VAT at 20% applies to alterations and extensions to homes in the UK. When a builder quotes you £60,000 + VAT, the cheque you write is for £72,000. That catches people out on every project.
How much does a single storey extension cost in the UK in 2026?
Let me give you two real examples.
The first is a flat roof extension measuring 4m x 4m. No work is planned inside the existing house, and an existing door in the side wall connects the new space to the home.
That project starts at £56,000 + VAT in 2026. It could easily pass £70,000 + VAT if you choose higher quality fittings, or if joining the extension to the house means forming a new structural opening in the external wall.
4m x 4m flat roof extension: from £56,000 + VAT in 2026
The second is a pitched roof extension measuring 4m x 5m, again with no work inside the existing house. That starts at £60,000 + VAT and can climb beyond £80,000 + VAT for the same reasons.
4m x 5m pitched roof extension: from £60,000 + VAT in 2026
Notice the conditions attached to those starting prices. No alterations to the existing house, and an existing opening doing the work of connecting old to new.
Very few projects stay that simple. The moment you knock through a wall, move a kitchen or re-plan the rooms around the extension, the budget moves with you.
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Wondering what your budget would actually build?
I run 30-minute Zoom consultations for homeowners across the UK. For £130 I will tell you whether your extension is affordable, feasible, or whether a better option exists for your home.
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How much does a wrap around extension cost?
A wrap around extension runs along the rear and side of a house, and it is one of the most popular ways to transform a family home.
A 45 square metre wrap around, with new openings formed in the existing external walls, starts at £165,000 + VAT in 2026. Add new kitchen units, large areas of glazing or high end internal finishes and it can exceed £200,000 + VAT. The glazing deserves a special mention. Large sliding or bifold doors are often the whole point of this type of project, and they are one of the fastest ways to consume a budget.
45 square metre wrap around extension: from £165,000 + VAT in 2026
How much does a two storey extension cost?
A two storey rear extension measuring 5m x 4m starts at £200,000 + VAT and can go beyond £300,000 + VAT.
5m x 4m two storey extension: from £200,000 + VAT in 2026
People are often surprised that doubling the floor area more than triples the price compared to a small single storey project. Here is why. A two storey extension rarely bolts onto the back of a house without consequence. The internal layout of both floors usually has to be altered so the new rooms integrate with the old ones, and a significant portion of the budget is spent inside the existing house rather than on the extension itself. That internal work is invisible in the estate agent photos, but you pay for it all the same.
Do these prices apply across the whole UK?
My figures come from projects built in Edinburgh, the Lothians and Fife. I design homes in that part of Scotland, so these are the numbers I can stand behind from first-hand experience.
Edinburgh has the most expensive housing market in Scotland, and one of the most expensive in the UK outside London. Yet the building gap is smaller than people assume.
Constructing an extension in Edinburgh costs about 10% more than in Fife, where I live. The difference is almost entirely labour, especially the skilled trades, because materials cost the same wherever you build.
That is worth remembering whenever you read a regional price guide. Labour rates move with location; bricks, timber and glazing do not. If you live in London or the South East of England, expect to pay more than the figures quoted here. Across much of the rest of the UK, my numbers are a realistic baseline, and the reasons budgets grow are the same everywhere.
Why is the starting price rarely the finishing price?
Every figure in this post is a starting price for a basic specification, and I have shown you the realistic upper number alongside it. The gap between the two is where extension budgets are won and lost.
The usual suspects are kitchens, glazing, bathrooms, floor finishes and structural work to the existing house. None of these are luxuries, and all of them are decisions you control.
I will be honest about the limits of a post like this. No article can price your project, because your ground conditions, your access, your specification and the availability of contractors in your area all move the number.
What a post like this can do is prevent you from starting a project with a budget that was never realistic. That mistake costs people thousands in abortive fees, and it is entirely avoidable.
A garden room extension I designed in Edinburgh. in 2026 this would cost at leat £6000 + VAT per square meter.
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Planning an extension in 2026?
Before you appoint a local architect or ask a builder for a price, book a 30-minute call with me. I will sense check your ideas, budget and timescale, and tell you what your project actually needs. I offer online consultations to homeowners right across the UK.
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