House Extension Cost Calculator UK
Get a realistic 2026 UK extension cost estimate in under a minute. Choose type, size, finish and region — we show low/mid/high figures with 10% fees and a 15% contingency built in, based on builder pricing across 519 UK towns cross-referenced with RICS BCIS. No email required.
How Much Will Your Extension Cost?
5 steps. Base £/m² by type × your floor area × finish multiplier × regional rate, plus ~10% professional fees and a 15% contingency buffer.
The Maths Behind Your Estimate
No black-box magic. We start with the 2026 UK base £/m² for your extension type, apply your finish and regional multipliers, then add professional fees and a contingency buffer.
Base Rates (£/m², by type)
- Single-storey rear — £2,000/m² (most popular)
- Single-storey side return — £2,100/m²
- Double-storey — £2,200/m² (footprint doubled across two storeys)
- Wraparound — £2,300/m² · Porch — £1,700/m²
- Loft conversion — £1,500/m²
Finish: Basic ×0.85 · Standard ×1.00 · Premium ×1.35 · Luxury ×1.70.
Fees, Contingency & Region
- +10% fees — planning, building regs, architect, structural engineer
- +15% contingency — recommended buffer for surprises
- London +35% · South East +15% · Midlands ±6–8%
- North East −15% · Northern Ireland −18% (lowest)
- Low/high band: mid ×0.88 to ×1.15 (matches real quote spread).
Indicative only, based on 2026 UK builder pricing across 519 towns + RICS BCIS. A real price needs a site survey.
Typical Extension Costs (2026, All-In)
base £/m² × size × finish + 10% pro fees + 15% contingency, Midlands baseline.
| Extension type & spec | Mid estimate | Cost range (all-in) |
|---|---|---|
| Single-storey rear, 20m², standard finish | £50,600 | £44,528–£58,190 |
| Side-return, 15m², standard finish | £39,848 | £35,066–£45,825 |
| Single-storey rear, 30m², premium finish | £102,465 | £90,169–£117,835 |
| Double-storey, 40m², standard finish | £111,320 | £97,962–£128,018 |
| Wraparound, 35m², premium finish | £137,474 | £120,977–£158,095 |
These figures power the calculator above — enter your own spec for a tailored estimate. For the full breakdown, read our complete cost guide.
Extension Costs at a Glance: 2026 Visual Guide
Where you build moves the budget more than any specification choice. The chart plots the mid-points of the 2026 regional ranges used across this page: a single-storey extension that prices at about £1,925/m² in the North East climbs to £3,125/m² in Greater London and £3,700/m² in inner and prime postcodes — roughly 90% above the cheapest English regions, before you have chosen a single finish.
Take the £50,600 mid estimate above for a 20m², standard-finish single-storey rear extension and split it the way a quantity surveyor prices a tender. The structural stages — groundworks, shell and roof — absorb around 47p in every pound, while professional fees (8%) and the 15% contingency built into every figure on this page take a fifth of the total before you spend anything on the parts you actually see.
On site, a straightforward single-storey extension runs to a rhythm every good builder knows. The programme below shows a typical 12-week build — the fast end of the 12–16 week range our calculator quotes. The milestone that matters is watertight at the end of week seven: once the roof is on and the glazing is in, weather can no longer stall the job and the internal trades can be booked with confidence.
Extension Cost by UK Region (2026)
The same job costs more in London and the South East and less in the North, Wales and Scotland, mostly through labour rates. The table shows BestBuilders’ 2026 regional cost model — national cost guides adjusted by a regional labour index and averaged across the towns in each region — for the main extension job types.
Indicative ranges, not quotes: your own price depends on specification, access and the installer’s workload. Get three itemised quotes to pin your figure down.
How we produced these figures
The cost ranges on this page come from BestBuilders’ 2026 regional cost model: published national cost guides adjusted by a regional labour index and averaged across 519 UK towns, then reviewed against published trade pricing. They are indicative ranges for a typical specification, not quotes — access, finish level and the builder’s workload move real prices either way.
Regulatory points (planning, Building Regulations, grants and VAT) are checked against current UK government guidance. Last reviewed: August 2026. Spotted something out of date? Tell us via the form above.
What homeowners asked us for — House Extension (last 90 days)
2,577 homeowners started a quote request for house extension through the BestBuilders quote tool between 9 Jun – 7 Sep 2026, UK-wide. These are their own answers to the tool’s questions, aggregated and anonymised — not a model, not an estimate.
Which type of extension do you require?
- 51% Rear extension
- 22% Side extension
- 10% Wraparound
- 7% Conservatory
1,367 answers
How many storeys is the extension?
- 73% 1 storey
- 24% 2 storeys
- 1% 3 storeys
- 0% 4 or more storeys
1,036 answers
What best describes the type of work you are wanting to be done?
- 88% Brand new extension
- 9% Replacement of existing extension
- 1% Repairs on existing extension
- 1% Other
267 answers
Which type of property is this for?
- 41% Semi-detached
- 29% Detached
- 18% Terraced
- 11% Bungalow
1,208 answers
How likely are you to make a hiring decision?
- 54% I'm planning and researching
- 17% I'm definitely going to hire someone
- 12% I'm likely to hire someone
- 10% I'm ready to hire now
795 answers
Where the enquiries came from
- 16% London
- 15% South East
- 15% North West
- 11% East of England
- 9% West Midlands
151 completed enquiries with a postcode we could place (ONS region)
Source: BestBuilders quote-tool answers, 9 Jun – 7 Sep 2026. Percentages are of the homeowners who answered each question; figures update daily. See the UK homeowner demand index for every trade.
What changes an extension quote the most
Single versus double storey is the first surprise: a two-storey extension is usually cheaper per square metre than a single-storey one, because the expensive parts — foundations, roof and groundworks — are shared across twice the floor area. If you are close to a budget ceiling, going up as well as out can be better value than a bigger footprint.
The costs that wreck budgets are underground. Foundations are priced on assumptions until the trenches are open, and made ground, a high water table, nearby trees or a sloping site can all mean deeper digs, piling or a raft — none of which shows in a headline per-square-metre figure. A ground investigation before you sign is cheaper than the argument later.
Above ground it is the glazing and the fit-out that move the number: bifolds, large rooflights and a new kitchen or bathroom inside the extension are where a shell becomes a room. On a terrace or semi the Party Wall etc. Act 1996 will usually apply too. If the extension is built around a new kitchen, price that separately with the kitchen renovation cost guide.
Frequently Asked Questions
How accurate is the UK extension cost calculator for 2026?
Does the calculator include VAT and planning fees in 2026?
Why does the calculator give a range, not a single price?
How much does a single-storey extension cost per m² in the UK in 2026?
How much does a two-storey extension cost per m² in the UK in 2026?
Does the calculator account for difficult-access sites in 2026?
How do I get accurate extension quotes after using the calculator?
Does the calculator work for wraparound and side-return extensions in 2026?
Do I need planning permission for an extension in 2026, and what does it cost?
Run a trade business? Your workshop, office or unit is on business rates — and out-of-contract business electricity typically costs 40–80% over a negotiated rate. Compare business energy prices — takes about 2 minutes, no broker call-backs.