House Extension Costs at a Glance (2026)
| Extension Type | Cost Per m² | Typical Size | Estimated Total |
|---|---|---|---|
| Single storey rear extension | £1,500–£2,200/m² | 15–25m² | £22,500–£55,000 |
| Double storey extension | £1,800–£2,500/m² | 25–40m² | £45,000–£100,000 |
| Side return (terraced) | £2,000–£3,000/m² | 8–15m² | £16,000–£45,000 |
| Wrap-around extension | £1,800–£2,800/m² | 25–50m² | £45,000–£140,000 |
| Garage conversion | £800–£1,500/m² | 15–25m² | £12,000–£37,500 |
These figures include groundworks, structure, insulation, windows, first and second fix — but not kitchen/bathroom fittings, flooring, decoration, architect fees, or planning costs.
What's Included in an Extension Quote?
- Groundworks and foundations
- Block or brick walls (cavity or solid)
- Flat or pitched roof structure and covering
- External windows and doors (standard spec)
- Insulation (floor, walls, roof)
- First and second fix carpentry
- Plasterboard and skim
- Electrics (lighting, sockets) and heating connection
- Drainage connections
Typically not included: architect/designer fees (£2,000–£8,000), planning application (£206 as of 2026), structural engineer (£500–£1,500), kitchen or bathroom fittings, flooring, decoration, and landscaping to make good the garden.
Planning Permission Requirements
Many extensions fall under Permitted Development (PD):
- Single storey rear extension up to 4m deep (detached) or 3m (semi/terrace) without prior approval
- Up to 8m (detached) or 6m (other) under the Larger Home Extension Scheme with neighbour consultation
- Must not exceed half the original garden area
- No higher than the existing roof
You need full planning permission for: double storey extensions, extensions within 2m of a boundary over 4m height, properties in conservation areas or AONBs, and anything exceeding PD limits.
All extensions need Building Regulations approval. Budget £600–£1,200 for Building Control fees.
What Drives Extension Costs Higher?
- Bifold or sliding doors — replacing a standard rear door with 3–4m bifolds adds £3,000–£8,000 to the project
- Flat roof lantern — a glazed roof lantern costs £2,000–£5,000 fitted
- Underfloor heating — wet UFH in a single-storey extension adds £2,000–£4,000
- Structural steelwork — opening up to the existing house often requires RSJ steels; each structural opening adds £800–£2,500
- Services relocation — if gas, electrics, or drainage need redirecting, each trade adds cost
- Party wall agreement — if extending within 3m of a shared boundary or party wall, legal fees of £800–£2,000 per neighbour are common
Self-Build Project Management vs Main Contractor
Main contractor — one builder takes responsibility for all trades, materials, and scheduling. Adds 15–25% to build cost in management margin but removes coordination burden. Strongly recommended if you've not managed a construction project before.
Self-managed — you hire groundworker, bricklayer, roofer, carpenter, plumber, electrician, and plasterer independently. Can save £5,000–£15,000 on a typical extension, but requires significant time investment and resilience when trades clash.
A middle ground: hire an architect or project manager for a 10–15% fee to manage on your behalf without their own trade markup.
Frequently Asked Questions
How much does a house extension cost per square metre in 2026?
Typical build costs run £1,500–£2,200/m² for a single storey rear extension and £1,800–£2,500/m² for a double storey. Side returns cost more per m² (£2,000–£3,000) due to restricted access.
Do I need planning permission for a rear extension?
Many single storey rear extensions fall under Permitted Development and don't need full planning permission. The standard limit is 4m deep for detached houses, 3m for semi-detached and terraced. All extensions need Building Regulations approval.
How long does a house extension take to build?
A single storey extension typically takes 8–14 weeks on site. A double storey takes 14–22 weeks. Allow 3–6 months before work starts for planning, drawings, and Building Regs approval.
Does a house extension add value?
Yes. Adding floor space typically adds 5–10% per additional room. A well-executed kitchen extension adding 20m² to a £350,000 home could add £25,000–£40,000 in value for a £30,000–£45,000 build cost.
What is a party wall agreement and do I need one?
A Party Wall Agreement is a legal document required when you build within 3–6m of a neighbour's wall or foundation, or work on a shared wall. A surveyor serves notice and draws up the agreement. Budget £800–£2,000 per adjoining owner.
Are You a Tradesperson?
These are consumer prices including labour and materials. If you're a builder working out what to charge, our free calculator covers your overheads, tax, and target income.
Calculate Your Day Rate