Extension and loft conversion costs, without the guesswork
See what your project will really cost – including design, building control, planning, party wall and VAT – then compare quotes from local builders.
Work out your cost
Choose your project, size, finish and region. The estimate updates as you go.
Extension & loft conversion cost calculator
Get an instant 2026 estimate for your project. Adjust the options to see how size, finish and location change the price.
| Building work (labour & materials) | |
| Kitchen fit-out | |
| Design, structural engineer & building control | |
| Planning / certificate fee | |
| Party wall surveyor(s) | |
| VAT | |
| Recommended budget incl. 10% contingency |
Guide prices for 2026 based on published UK cost data and regional adjustments. Your actual price depends on your property, access, ground conditions and specification. How we calculate this.
Average costs in 2026
Typical all-in prices for the most common projects, including fees and VAT.
| Project | Typical size | Typical all-in cost (inc. VAT) | Build cost per m² (ex VAT) |
|---|---|---|---|
| Single storey extension | 20m² | £55,000 – £73,000 | £1,800 – £3,500 |
| Kitchen extension (with new kitchen) | 25m² | £87,000 – £125,000 | £1,800 – £3,500 |
| Double storey extension | 40m² (2 × 20m²) | £97,000 – £125,000 | £1,600 – £3,100 |
| Side return extension | 15m² | £47,000 – £63,000 | £2,000 – £4,000 |
| Velux loft conversion | 25m² | £23,000 – £35,000 | £610 – £1,170 |
| Dormer loft conversion | 30m² | £46,000 – £70,000 | £1,090 – £2,090 |
| Hip to gable loft conversion | 30m² | £56,000 – £80,000 | £1,350 – £2,430 |
| Mansard loft conversion | 35m² | £72,000 – £108,000 | £1,480 – £2,870 |
| Garage conversion | 15m² | £13,000 – £20,000 | £480 – £1,230 |
Cost guides by project
A dedicated, sourced guide for every type of extension and loft conversion.
Single storey extension
Prices per m² and by size, from 3m × 3m to 30m².
Kitchen extension
Build plus kitchen: what a kitchen diner really costs.
Two storey extension
Why a double storey costs less per m².
Side return extension
Victorian terrace side infills, steel and party walls.
Wrap-around extension
L-shaped side and rear extensions, costed.
Conservatory
Lean-to, Victorian and P-shaped prices.
Orangery
Lantern roofs, brick piers and per m² costs.
Garden room
Insulated garden offices and studios.
Dormer loft conversion
The UK’s most popular loft conversion.
Hip to gable loft
For semis and detached houses with hipped roofs.
Mansard loft conversion
The biggest loft space, and the priciest.
Garage conversion
Single and double garages turned into rooms.
More guides: House extension costs (all types) · Loft conversion costs (all types) · Loft conversion calculator · Rear extension · Side extension · Flat roof extension · Lean-to conservatory · Conservatory roof replacement · Porch · Basement conversion · Bungalow loft conversion · Loft conversion cost London · Permitted development rules · Lawful development certificate · Party wall agreements · Financing an extension
What is included in the cost of an extension?
Most online prices only quote the building work. Our calculator and guides include everything you are likely to pay for:
- Building work – labour and materials from foundations to plastering, including windows, doors, electrics, plumbing and heating.
- Design fees – an architect or architectural technician, typically 4–7% of build cost.
- Structural engineer – calculations for steel beams and foundations, usually £500–£2,500.
- Building control – £600–£1,200 for inspections and the completion certificate.
- Planning or certificate fee – £548 for a householder planning application, or £274 for a lawful development certificate.
- Party wall surveyors – £1,000–£3,000 per neighbour, if needed.
- VAT at 20% on building work and most fees.
- Contingency – we recommend adding 10% for unexpected costs.
Read how we calculate our estimates for the full methodology and sources.
Why get several quotes?
Quotes for the same extension can vary by 30% or more depending on the builder’s workload, overheads and how they price risk. Comparing at least three detailed, itemised quotes is the simplest way to avoid overpaying – and to spot a quote that is suspiciously low.
Get free quotes from local builders
Tell us about your project and we will put you in touch with vetted local builders and loft specialists. Free and no obligation.
Frequently asked questions
How much does a 4m × 4m extension cost?
A 16m² single storey extension typically costs £44,500–£59,000 all-in, including fees and VAT, outside London. In London expect around £56,500–£74,000.
Is it cheaper to extend or convert the loft?
Per square metre, a loft conversion is usually cheaper because the structure – walls and roof – already exists. A dormer loft conversion costs around £1,100–£2,100 per m² compared with £1,800–£3,500 per m² for a single storey extension.
Do I pay VAT on an extension?
Yes. Extensions and loft conversions on existing homes are charged VAT at the standard 20% if your builder is VAT-registered. Planning application fees are not subject to VAT.
How accurate is the calculator?
It gives a realistic guide range based on published 2026 UK cost data, adjusted for region and specification. Your actual price depends on your property, access, ground conditions and finishes, so always get detailed quotes before you commit.