Single storey extension cost

Size is the biggest single factor in what you pay, because almost every part of the job, from foundations and walls to roofing and plastering, grows with the floor area. MyJobQuote, updated in October 2026, says a small basic extension 'can cost as little as £15,000', while a large, high quality one 'could cost up to £100,000'. Its figures below show how the typical range widens as the extension gets bigger and as the specification rises from basic to high.

Treat these as national guide prices rather than a quote. MyJobQuote gives no South East figure, but it does say that London and the South East cost more than the rest of the country. Its page also lists architect, structural engineer and party wall costs as extras on top of these totals, so they are not the whole budget.

The shape of the extension matters as well as its size. A side return usually involves working tight to a boundary and often to a party wall, while a wrap around combines a rear and side extension and needs more structural steel to open up the existing house. MyJobQuote and Less both price these types separately, and Homebuilding and Renovating gives its own side return rates, which sit above its general single storey figures.

• Bi-fold doors: £3,000 to £6,000 (Less.co.uk)
• Roof lantern: £1,500 to £3,500 (Less.co.uk)
• Underfloor heating: £50 to £80 per m² (Less.co.uk)
• Fitted kitchen: £8,000 to £20,000 or more (Less.co.uk)
• Difficult ground: adds £2,000 to £8,000 (Less.co.uk)
• Restricted access: adds 5 to 10% (Less.co.uk)

Beyond size and type, the specification of glazing, the roof design, the ground conditions and how easily materials and machinery can reach the back of the house all move the price. Less lists these extras on top of its rates per square metre:

Many single storey extensions to houses in England can be built under permitted development, which means no planning application is needed, but only if every limit is met. The Planning Portal sets out these main limits for a single storey extension:

• Extensions and other buildings cannot cover more than half the area of land around the 'original house'.
• A rear extension must not extend beyond the rear wall of the original house by more than four metres for a detached house, or three metres for any other house, and must not be more than four metres high.
• Within two metres of a boundary, the eaves cannot be higher than three metres.
• A side extension must be single storey, no more than four metres high and no wider than half the width of the original house.
• Materials should be similar in appearance to the existing house.

A larger rear extension of up to eight metres for a detached house or six metres for other houses is possible through prior approval, except on designated land or a SSSI. GOV.UK's guidance says you must notify the council and cannot start until it confirms prior approval is not required, or 42 days pass without a decision, and the GOV.UK fee for this prior approval is £249. If you need full permission, the householder application fee in England is £548 from 1 April 2026, and a lawful development certificate confirming a proposed project is lawful costs half the normal planning fee. These rules apply to houses only, not flats or maisonettes, and they are tighter in conservation areas and other designated land, where an Article 4 Direction applies, and for listed buildings, which need listed building consent. Always check with your local planning authority before relying on permitted development, and read our guide to extension planning permission. No one can promise that permission will be granted.

Building regulations approval is needed regardless of planning, because GOV.UK says the regulations cover 'the construction and extension of buildings'. You can apply to the council or use a private registered building control approver. A full plans application gets a decision within five weeks, or two months with your consent, while a building notice lets smaller projects start two days after it is submitted, without formal approval of plans.

MyJobQuote gives typical build durations and labour costs for the main types of single storey extension. These cover the build itself, so allow extra time beforehand for drawings, any planning or prior approval decision, building control and party wall notices.

Quotes are only comparable when every builder prices the same drawings and the same specification. Ask each one for an itemised quote that states whether VAT is included, which items are provisional sums, and what is left out, such as the kitchen, flooring, decoration, skips or scaffolding. Check who will handle building control and party wall notices, how payments are staged against completed work, and what written contract and insurance the builder offers. Homebuilding and Renovating suggests a contingency of at least 10%, plus a further 5% for inflation, which is worth building into your budget before you choose.

To compare prices from builders who work in your area, fill in our short form or call 01273 115520 to get up to 3 free quotes from checked local builders, with no obligation. You can also try our extension cost calculator for a first estimate, or read more about extensions.