House Extension Cost UK 2026: single, double, side-return, wrap-around and kitchen
What a house extension should cost in 2026, by type and by region, with the spec choices that move the price and the fees most quotes leave out.
Key facts
- Single-storey rear extension: £2,200–£3,200/m² UK, £3,000–£4,200/m² London.
- Two-storey rear extension: £2,400–£3,400/m² UK, £3,400–£4,800/m² London.
- Side-return extension (London terraces): £3,200–£4,000/m².
- Wrap-around (side + rear): £3,600–£4,800/m² in London.
- Kitchen extension: build cost above, plus £15,000–£60,000 for kitchen and appliances.
- Professional fees and VAT typically add 25–35% on top of the build cost.
Extension cost is one of the most-searched and worst-answered questions in UK construction. Most online calculators give you a single number with no sense of what is inside or excluded. This guide gives the working 2026 ranges by type and region, what they include, and the variables that move you up or down.
A price is not a quote. A quote is not a contract. A contract is not a closing out the build. Most overruns happen between the first and the last.
House extension cost by type, 2026
Construction cost only, in £/m² of new floor area. Multiply by your gross internal area to get the build cost. Add 25–35% for fees, party wall, VAT, FF&E to reach the total project cost.
Single-storey rear extension
£2,200–£3,200/m² outside London. £3,000–£4,200/m² in Greater London. The default extension for terraced and semi-detached homes. Most common at 20–40m².
Two-storey rear extension
£2,400–£3,400/m² outside London. £3,400–£4,800/m² in London. Cheaper per m² than single-storey because the foundation and roof costs spread across two floors.
Side-return extension (London)
£3,200–£4,000/m² in London terraces. Filling the side alley of a Victorian terrace. Higher per m² than a clean rear extension because of party wall complexity, awkward access, and constrained working area.
Wrap-around (side return plus rear)
£3,600–£4,800/m² in London. Combines a side return with a rear extension into a single L-shape. The most spec-sensitive extension type; glazing alone can swing the price by £20,000.
Double-storey side extension
£2,600–£3,600/m² nationally. Adds a new flank to a semi or detached home. Planning sensitivity is high; conservation areas often refuse them.
Kitchen extension
Same per-m² rates as the underlying extension type, plus £15,000–£60,000 for the kitchen itself (cabinets, worktops, appliances, tiling, lighting). A £40,000 mid-range kitchen with integrated appliances is typical in London.
What moves the price inside the range
Five variables explain most of the spread between two builders pricing the same drawings:
- Glazing spec. Standard aluminium sliders: included. Crittall or steel-framed: £4,000–£12,000 extra. Frameless structural glass: £15,000+.
- Kitchen and bathroom specification. Trade-spec vs designer-spec can double the FF&E budget on the same floor area.
- Ground conditions. Tree roots, made-up ground, high water table, asbestos drains. All discovered after the quote.
- Structural openings. Removing a load-bearing wall to open up to the existing kitchen typically adds £6,000–£14,000.
- Party wall complexity. One difficult neighbour can add £5,000+ and weeks of programme. See our party wall surveyor cost guide.
What the headline price excludes
Builders quote build cost. Homeowners pay total project cost. On a £150,000 London extension, the gap is typically £40,000–£60,000:
| Item | Typical 2026 cost (London, £150k build) | | --- | --- | | Architect fees (design 1–5) | £12,000–£18,000 | | Structural engineer | £2,500–£5,000 | | Party wall (per neighbour) | £1,500–£3,500 | | Building control | £900–£1,800 | | Planning (if needed) | £258 fee plus drawings | | Independent cost expert | £3,500–£8,500 | | Kitchen and FF&E | £20,000–£50,000 | | VAT on construction | £30,000 | | Total over headline | £45,000–£75,000 |
How much contingency to hold
- 10–15% on extensions to older properties where fabric is being opened up.
- 7–10% on clean new-build extensions with no existing structure at risk.
- Anything below 7% is optimistic. The single largest cause of homeowner overrun is no contingency at all.
Common extension cost mistakes
- Accepting the cheapest quote. The cheapest quote almost never becomes the cheapest closing out the build. Our guide to why most building quotes are wrong explains the mechanism.
- No measured pricing document. Three builders quoting from drawings alone will price three different scopes. A measured pricing document forces them to price the same scope.
- Builder-supplied PC sums for kitchens, bathrooms and joinery. These almost always undershoot. Get a real kitchen quote first.
- No retention. Hold 5% until 6 months after practical completion, half released at PC, half at end of defects.
Should you appoint a cost expert?
For extensions under £100,000, a cost-checked schedule of works and a clean your build contract Minor Works contract usually covers it. Above £150,000, an independent cost expert pays for itself. Above £300,000, it is essential. The role and the fees are explained in our pillar guide what does a cost expert do.
Our extension and refurbishment costing service and builder quote audit are built around the most common homeowner scenarios.
Related reading
- Loft conversion cost UK 2026, the upstairs equivalent.
- Extension cost per m² in London, London-only £/m² breakdown.
- Side return extension cost (London), the side-return deep dive.
Get a fixed-fee proposal for your extension via our fee proposal form. We reply within one UK working day.
Frequently asked
- How much does a 30m² single-storey extension cost in 2026?
- Outside London: £66,000–£96,000 in build cost, typically £85,000–£125,000 all-in. In Greater London: £90,000–£126,000 in build cost, typically £115,000–£165,000 all-in. The wide range reflects glazing and finish spec, not size.
- How much does a two-storey extension cost?
- A 40m² two-storey rear extension is typically £96,000–£136,000 in build cost outside London, £136,000–£192,000 in London. Two-storey is roughly 10–15% cheaper per m² than single-storey because foundations and roof are shared across two floors.
- Are extension costs higher than two years ago?
- Yes, by roughly 8–12% versus 2024. Labour has driven most of the increase; materials have stabilised since the 2023 peak. Programme risk has fallen; price risk has not.
- What is included in the per-m² figure?
- Construction cost only: contractor's price including labour, materials, prelims and OHP. Excluded: VAT, professional fees (architect, structural engineer, cost expert, planning), party wall, building control, FF&E, kitchen appliances supplied by homeowner.
- Do I need planning permission for an extension?
- Most single-storey rear extensions in England fall under Permitted Development (up to 6m on detached, 4m on semi/terraced, lower in conservation areas). Two-storey extensions usually need full planning. Always confirm with your local authority before quote.
- Do I need a cost expert for an extension?
- For projects under £100,000, a cost-checked schedule of works and a robust your build contract Minor Works contract usually covers it. Above £150,000, an [independent cost expert](/homeowners/insights/what-does-a-quantity-surveyor-do) almost always saves multiples of the fee. Above £300,000, it is essential.
- How much contingency should I hold?
- 10–15% on extensions to older properties where you are opening up existing fabric. 7–10% on new-build extensions on a clean slab. Anything below 7% is optimistic and almost always blown.
- Is VAT charged on extensions?
- Yes, at 20% in almost all cases, because the work is on an existing home. The narrow exceptions (empty home over 2 years, listed building approved alterations) rarely apply. Our [VAT on home building work](/homeowners/insights/vat-on-home-building-work) guide explains.
