Rear Extension Cost UK: 2026 Prices by Size, Depth and Spec

What a rear extension costs in 2026

Rear extensions are priced by floor area, and in 2026 the standard-spec rate sits at £1,900–£2,600 per m², with basic builds from £1,700 and premium glass-led schemes up to £3,500. A 3m x 4m addition therefore starts around £23,000, while a 6m-deep kitchen-diner across the full width of the house can pass £70,000 once glazing and fit-out are counted.

The quirk worth knowing: a two-storey rear extension is better value per square metre. The blended rate is £1,700–£2,400 per m² because the expensive parts — foundations and roof — are shared by two floors. If you might ever want the extra bedroom, building both storeys at once is far cheaper than coming back later. Our single storey cost guide covers the one-floor option in detail, and the extension cost per m² page shows how the rate breaks down element by element.

Permitted development: the 3m, 4m, 6m and 8m rules

The permitted development rules in England are built around rear extensions, so most projects avoid a full planning application:

• 3m beyond the original rear wall on a semi-detached or terraced house, or 4m on a detached, without a full application
• 6m and 8m respectively under the prior approval route, where the council notifies your neighbours before signing off
• Maximum height of 4m for the single storey element
• A two-storey rear extension can go 3m beyond the rear wall but must stay at least 7m from the rear boundary

None of this applies in conservation areas or on listed buildings, and councils can remove rights locally, so check your address on the Planning Portal before committing to a design. Building regulations apply to every habitable extension regardless of the planning route.

Rear extension costs by size (2026)

Standard-spec totals outside London, excluding a new kitchen (£10,000–£25,000 extra). London and South East projects typically run 20–30% higher.

What moves a rear extension quote up or down

Size sets the baseline, but five items decide whether you land at the bottom or top of the range:

• Drains: the back of most houses is exactly where the foul drain runs, so expect a diversion or a build-over agreement with the water company
• The opening: a full-width steel to merge old and new space needs engineer design and temporary propping — wider means dearer
• Glazing: bifold or sliding doors add £3,000–£8,000, and roof lanterns sit on top of that
• Kitchen and bathroom: a relocated kitchen adds £10,000–£25,000 and a new bathroom £4,000–£8,000
• Roof choice: a warm flat roof is the value option; parapets, pitched roofs and lanterns all add cost

If you are tempted to extend sideways at the same time, see how a wrap-around extension changes the economics.

Party walls, fees and the 10–15% people forget

On a terrace, a rear extension often triggers the Party Wall Act with both neighbours, at £700–£1,500 each for the surveyor and award. A structural engineer for the steel and foundations costs £600–£1,400. Professional fees overall — architect, engineer and surveys — add 10–15% to the build price, and a 10–15% contingency on top is the difference between a stressful project and a ruinous one. Builder quotes to homeowners normally include VAT; get that confirmed in writing.

Timescales and getting comparable quotes

Single storey rear extensions take 8–12 weeks on site; two-storey versions run 10–14 weeks. Add two to four months up front for design, engineering and either a lawful development certificate or planning decision.

Use the rear extension cost calculator to get a benchmark price for your size and spec, then collect three itemised quotes from local extension builders against the same drawings so any price gap is explained by the builder, not the assumptions.

FAQs: Rear Extension Cost UK