Average Cost of a Detached Extension in 2026 (UK)
A detached extension — a garden room, studio or self-contained annexe standing apart from the house — costs roughly £1,300–£2,100 per m² as an insulated garden room, or £2,200–£3,200 per m² as a fully serviced annexe in 2026. A typical 25m² detached room lands around £33,000–£52,000, while a two-room annexe with its own kitchen and bathroom runs £68,000–£95,000+. Because the structure stands on its own you avoid knocking through the existing house — but you pay for a full base, roof and services from scratch. The final figure hinges on size, whether you add plumbing, the cladding and your region. Here's the full breakdown.
Detached extension cost by size — 2026
Insulated garden-room specification, finished; annexe fit-out adds a kitchen and bathroom. Source: BestBuilders 2026 quote data · 519 UK towns
Detached extension cost by size (2026)
| Size | Insulated garden room | Self-contained annexe (kitchen & bath) |
|---|---|---|
| 12m² (office/studio) | £16,000–£26,000 | £32,000–£45,000 |
| 20m² | £26,000–£42,000 | £48,000–£68,000 |
| 30m² | £38,000–£60,000 | £68,000–£95,000 |
| 40m² (two-room annexe) | £50,000–£78,000 | £88,000–£125,000 |
Ranges assume a timber-framed, insulated structure on a standard base with a warm flat roof. A brick-and-block build adds roughly 15–25%. London and the South-East typically sit 12–30% above these figures — see the regional table below.
Types of detached extension
"Detached extension" covers everything from a simple home office at the bottom of the garden to a fully independent annexe. What you choose sets the cost per m² more than anything else.
| Type | Best for | Typical cost per m² |
|---|---|---|
| Garden office / studio | Working from home; no plumbing | £1,300–£1,900 |
| Garden room / gym / hobby room | Year-round use, sometimes with a WC | £1,500–£2,200 |
| Self-contained annexe (granny annexe) | Independent living: kitchen, shower room, sleeping | £2,200–£3,200 |
| Masonry (brick & block) outbuilding | Matching the house; longest lifespan | £1,900–£2,800 |
Timber-framed rooms are quicker and cheaper and suit garden offices and gyms. Masonry costs more and takes longer but lasts decades and matches a brick house. The moment you add a kitchen or a bathroom you move into annexe territory, where drainage, heating and fit-out push the price up sharply.
Cost by type and region
The specification decides most of the price. Here's what each level buys per m² in 2026:
| Specification | Cost per m² (2026) | What you get |
|---|---|---|
| Garden office shell | £900–£1,300 | Insulated, clad, powered — no internal fit-out |
| Insulated garden room (finished) | £1,300–£2,100 | Plastered, wired, floored, heated, decorated |
| Garden room with WC / shower | £1,800–£2,500 | Adds drainage and a shower room |
| Self-contained annexe | £2,200–£3,200 | Kitchen, bathroom, heating, full fit-out |
| Masonry (brick & block) uplift | +£250–£500 | Over timber frame — longer lifespan |
| Poor-ground groundworks | +£2,000–£6,000 | Piled or reinforced base where the ground is soft |
| Utility connection to the house | +£1,500–£5,000 | Trenched power, water and drainage runs |
| Architect / technician drawings | £750–£2,500 | See architect costs |
Cost by region
Labour rates move the total more than materials. As a rough guide against the UK average:
| Region | Typical variance vs UK average |
|---|---|
| London | +20% to +30% |
| South East | +12% to +20% |
| South West & East of England | +5% to +12% |
| Midlands | Around the UK average |
| North of England | −5% to −10% |
| Scotland & Wales | −5% to −8% |
| Northern Ireland | −10% to −15% |
What's in the price — and what isn't
A headline quote for a detached extension usually covers the shell and the basics. The extras are where budgets slip, so price them at design stage.
Usually included
- Base and foundations for standard ground
- Timber frame or masonry walls
- Warm flat or pitched roof and covering
- Insulation, windows and external doors
- Basic power: sockets, lighting, a consumer unit
Often extra
- Plumbing and drainage runs for a WC, shower or kitchen
- Kitchen or bathroom fit-out
- Decking, paths or landscaping around the building
- Upgraded foundations on soft or sloping ground
- Skip hire and waste disposal
Removing an existing structure
If a tired shed, old garage or hardstanding has to come out first, demolition, clearance and skip hire typically add £800–£3,000 depending on size and whether there's asbestos in an old garage roof, which needs licensed removal.
The one figure no one can price up front is the ground. Until the footings are dug you don't know what's under the lawn — hold a 10–15% contingency.
Detached vs attached extension: the real difference
A detached build avoids the most disruptive and expensive parts of an attached extension — there's no structural knock-through, no steel beam into the existing house and no making-good of the rooms you opened up. It can also often go up under permitted development. Against that, you pay for a complete independent base, roof and utility runs rather than borrowing them from the house. As a rough comparison, a detached garden room runs £1,300–£2,100 per m² against £2,000–£2,600 per m² for an attached single-storey extension — but an attached extension adds directly connected living space and usually returns more at resale. If you need an extra bedroom or a bigger kitchen, attached is often worth the premium; if you need a home office, gym or annexe, detached is usually the smarter spend. See our single-storey extension costs and rear extension costs to compare.
How long it takes
- Prefab or timber-framed garden room — 1–3 weeks on site once the base is down, plus manufacturing lead time.
- Bespoke insulated garden room with a WC — 3–5 weeks including drainage.
- Masonry self-contained annexe — 8–14 weeks: groundworks, walls, roof, first and second fix, kitchen and bathroom, connections and sign-off.
Add 2–8 weeks up front for design, a planning check and any Building Regulations submission before anyone is on site.
How long it lasts
- Timber-framed garden room — 20–30 years for the structure; the cladding usually needs attention sooner.
- EPDM or GRP flat roof covering — 20–25 years, often with a matching guarantee.
- Masonry annexe — 50+ years, on a par with the house itself.
What fails first is almost always the roof covering and the external cladding or decking, followed by window and door seals. A well-detailed warm roof and a maintainable cladding choice are worth more over the life of the building than a cheaper headline price.
Planning permission and building regulations
Planning permission. A detached outbuilding used as an office, gym or hobby room is often allowed under permitted development in England: single storey, a maximum eaves height of 2.5m and an overall height of 3m, or 4m with a dual-pitched roof, and no more than 2.5m high if it's within 2m of a boundary. It must not cover more than 50% of the land around the original house. Crucially, permitted development does not cover a building used as separate, self-contained living accommodation — a true annexe someone lives in independently usually needs a full planning application. Conservation areas, listed buildings and Article 4 areas have tighter rules, so always confirm with your local planning authority.
Building regulations. A detached building under 15m² with no sleeping accommodation is normally exempt. Between 15m² and 30m² it's often exempt too, provided it's more than a metre from any boundary or built substantially from non-combustible materials. Anything over 30m², anything closer to a boundary, and any building with sleeping accommodation needs Building Regulations approval — which is signed off by Building Control, not the planning department.
Hiring checklist
- Ask to see two recent detached builds you can visit or phone about — garden rooms and annexes, not just general building work.
- Ask to see their public liability insurance and check the level of cover yourself.
- Make sure the quote names the base type, wall build-up, roof covering and insulation — not just a lump sum.
- Confirm who handles the planning check and any Building Regs, and whether it's in the price.
- Agree a written stage-payment schedule — never more than 10–15% up front.
- Get three itemised quotes on the same brief so the numbers are genuinely comparable.
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