Adding an extension is one of the most popular ways UK homeowners create more space — without the disruption and cost of moving. But it's also one of the biggest financial commitments a homeowner can make. Understanding the real costs before you commit to a design is essential.
This guide covers house extension costs in the UK for 2025, by type and size, plus what drives prices up — and how to manage the process effectively.
House Extension Costs at a Glance
Extension costs vary significantly depending on type, size, specification, and location. Here's a broad overview:
| Extension Type | Typical Cost Range |
|---|---|
| Single-storey rear extension | £20,000 – £50,000 |
| Double-storey extension | £40,000 – £100,000+ |
| Side return extension | £15,000 – £40,000 |
| Wrap-around extension | £50,000 – £120,000+ |
London and the South East command a premium — typically 20–30% above the national average. Rural locations may come in under these ranges.
Cost Per Square Metre
A useful benchmark is cost per square metre of floor area added. In the UK, expect:
- Budget spec: £1,500 – £1,800/m²
- Mid-range spec: £1,800 – £2,500/m²
- High-end / premium: £2,500 – £3,500/m²
A 20m² single-storey kitchen extension at mid-range spec would therefore cost in the region of £36,000–£50,000 — before any interior fit-out.
Single-Storey Rear Extension (£20,000 – £50,000)
The most common extension type in the UK, typically used to enlarge a kitchen or create a kitchen-diner or garden room. Costs depend on:
- Size — the bigger the footprint, the higher the cost
- Roofing type — a flat roof is cheaper than a pitched or lantern roof
- Glazing — standard windows vs. bi-fold or sliding doors
- Internal fit-out — is new kitchen installation included?
A modest 15m² flat-roof extension at budget spec might come in at £22,000–£28,000. Add bi-fold doors, a lantern roof, and underfloor heating, and the same footprint can exceed £45,000.
Double-Storey Extension (£40,000 – £100,000+)
A double-storey extension adds space on both floors — commonly used to add a bedroom and en suite upstairs while extending the kitchen below. Although costs are higher, the cost per square metre is often lower than a single-storey, since the roof and foundations are shared across both floors.
Key variables include:
- Whether party wall agreements are needed (if building close to a boundary)
- The specification of the upstairs bathroom or bedroom
- Whether staircase reconfiguration is required
Side Return Extension (£15,000 – £40,000)
Common in Victorian and Edwardian terraced houses, side return extensions fill in the unused alleyway alongside the back of the house. They're typically narrower than a rear extension but can dramatically transform a kitchen.
Costs are generally lower due to the smaller footprint, though the restricted access can push up labour costs slightly.
Wrap-Around Extension (£50,000 – £120,000+)
A wrap-around combines a rear and side return extension to create an L-shaped ground-floor addition. It delivers the greatest space gain on the ground floor but is also the most expensive and the most likely to require full planning permission.
What's Included — and What Adds Cost
Standard extension quotes usually include:
- Groundworks and foundations
- Structural frame and external walls
- Roofing (flat felt or GRP)
- Standard windows and a door
- Plasterwork and first-fix electrics
- Basic decoration
What typically adds cost:
- Bi-fold or sliding doors (add £3,000–£10,000 depending on size)
- Roof lantern (add £2,000–£8,000)
- Underfloor heating (add £1,500–£4,000 for a 20m² area)
- Kitchen installation (add £5,000–£25,000+)
- Structural steel (if removing internal walls — add £1,500–£5,000)
- Listed building or conservation area requirements
Planning Permission: Do You Need It?
Many single-storey rear extensions fall under Permitted Development (PD) rights — meaning you don't need full planning permission. Under PD rules (England), a single-storey rear extension can be built without permission if it:
- Extends no more than 4 metres from the rear wall (detached house) or 3 metres (semi or terrace)
- Is no taller than 4 metres, or the height of the existing eaves
- Covers no more than half the garden area
However, PD rights don't apply to:
- Listed buildings — always need planning permission
- Conservation areas — PD rights are more restricted
- Flats and maisonettes — PD rights don't apply
Double-storey extensions and wrap-arounds almost always require full planning permission. Always check with your local planning authority or use the Planning Portal before starting work.
Party Wall Agreements
If your extension is being built on or close to the boundary with a neighbouring property, you may need to serve a Party Wall Notice under the Party Wall Act 1996. This applies when:
- You're building on the boundary line
- Excavating within 3 metres of a neighbour's foundations
Your neighbour has the right to appoint a surveyor, and costs for party wall matters can range from £700 to £2,000+. Factor this in early.
What Return on Investment Can You Expect?
A well-executed extension typically adds 10–15% to the value of a UK property, sometimes more in high-demand areas. A kitchen-diner extension is particularly valued by buyers. That said, ROI depends on the local market — in some areas, you can over-capitalise.
Always benchmark against comparable sold prices in your street before committing to a high-spec build.
For a full guide to what you can build without planning permission — including precise single-storey, two-storey, and outbuilding limits — see our Permitted Development Rights UK guide. For the kitchen planning and workflow involved in extending your home, see our kitchen renovation planning guide. For a comprehensive 2025 cost breakdown by extension type, regional benchmarks per square metre, and a full list of professional fees, see our home extension costs UK 2025 guide.