How Much Does an Extension Cost in 2026?

The price per square metre covers the building work: foundations, walls, roof, windows and doors, insulation, plastering, electrics and heating, finished ready to decorate. Fees, a kitchen or bathroom and a contingency come on top, and the fees section adds them up. Basic means standard materials and a simple box shape; high means larger glazing, bespoke joinery and better finishes. Multiply the rate by the floor area you are adding, measured inside. For a double storey, add both floors together.

Only MyJobQuote prices extensions by spec, so the standard range comes from all five guides and the basic and high ranges keep MyJobQuote's own spacing either side of it. The next section shows every figure.

Three of these guides come from websites that match homeowners with tradespeople (MyJobQuote, Rated People and Bark), one from a builders' trade body and one from a homeowners' advice group quoting Checkatrade. MyJobQuote is the most recent. Rated People's totals work out dearest per square metre for small extensions and cheapest for large ones, which is why its range is so wide.

A double storey extension rarely costs twice as much as a single storey one with the same footprint. The ground floor, the foundations and the roof are the costly parts, and the upper floor shares them. Against that, a double storey needs a staircase landing or a new opening upstairs, more scaffolding and nearly always a planning application, because permitted development only allows it to come 3 metres out from the back wall.

The guides' own examples land inside these ranges. HomeOwners Alliance puts a 20 m² single storey extension at £40,000 to £56,000 and 30 m² at £60,000 to £84,000; MyJobQuote puts a typical single storey of 20 to 30 m² at £44,000 to £60,000 and a double storey of 40 to 60 m² at £80,000 to £120,000. Use the extension cost calculator with your own floor area for a closer figure.

The other guides price the whole job. MyJobQuote puts a kitchen extension at £25,000 to £100,000, with an average of £62,500, against £52,000 for its average single storey extension. Bark prices a 20 m² kitchen extension at £26,400 to £50,000 before VAT, which is its rate per square metre times 20, so it looks like the building without the kitchen. MyJobQuote adds that appliances and fittings cost £3,000 to £10,000 and a kitchen fitter £1,500 to £4,000.

Most kitchen extensions open up the back of the house, so budget for a steel beam where a wall comes out, which is why a structural engineer is on every budget below. Large glazed doors are popular in kitchen extensions, but the insulation rules limit the glass, as the energy section explains.

A side return fills the narrow passage beside the back of a terraced or semi-detached house, usually to widen a kitchen. A lean-to has a roof sloping down from the house wall. A wrap-around combines a side return and a rear extension in an L shape, which gives the most ground floor space but usually means a planning application, because side extensions of more than one storey, and any side extension on protected land, fall outside permitted development.

• Where you live. MyJobQuote puts labour in London and the surrounding areas 10 to 25% above the national average, and HomeOwners Alliance puts a single storey extension in London and the South East at £2,300 to £3,400 per m². We do not adjust the calculator for region yet, because we only do that from official pay data.
• Size. Each square metre adds less than the last in Rated People's prices, because the design, the connection to the house and the site set-up are paid for once.
• Glazing. MyJobQuote's prices leave out bifold doors, and the insulation rules cap the glass at 25% of the extension's floor area unless the rest is built better.
• The ground and the drains. Poor ground means deeper foundations, and a drain under the extension may have to be moved. MyJobQuote puts a drain survey at about £250 and drain work at £200 to £1,300.
• VAT. Building work on an existing home carries 20% VAT. Bark notes a 5% rate only in narrow cases, such as converting a building that was not a home. Ask whether each quote includes it.

Ask every builder which of these the quote includes. Some firms include drawings, building control and scaffolding and some do not, which makes two quotes for the same extension look further apart than they are.

Every extension under permitted development must also meet these limits:

• Extensions and other buildings, sheds included, cover no more than half the land around the original house.
• Nothing is higher than the highest part of the roof, and the eaves are no higher than the existing eaves. Within 2 metres of a boundary, the eaves can be no more than 3 metres high.
• Nothing comes forward of the front of the house, or of a side that faces a road.
• Materials look similar to the house, and there are no verandas, balconies or raised platforms.
• Upstairs side windows are obscure glazed and fixed shut, unless the part that opens is more than 1.7 metres above the floor, and a roof of more than one storey matches the pitch of the house where practical.

Permitted development does not apply to flats or maisonettes. On protected land (a conservation area, a National Park, an Area of Outstanding Natural Beauty, the Broads or a World Heritage Site) every side extension and every rear extension of more than one storey needs an application, the larger rear extension route is closed and cladding is not allowed. Some councils have removed these rights in particular streets with an Article 4 direction, so check with yours.

Prior approval is lighter than a full application: the council consults the neighbours next door and considers the effect on their homes. A lawful development certificate costs half the application fee, £274, and gives you proof that the work was lawful, which helps when you sell. Government planning fees rise each 1 April (GOV.UK fee indexation). These rules are for England. Wales sets different limits, and Scotland and Northern Ireland have their own systems.

The Planning Portal says most extensions need building regulations approval. Building control checks the foundations and structure, fire safety, drainage, ventilation and the insulation of the new walls, roof, floor and glass. You can use your council's building control team or a registered building control approver; HomeOwners Alliance puts the fee at about £300 to £500, and every council publishes its own charges. Keep the completion certificate: it is the proof that the work was signed off.

The Party Wall etc. Act 1996 applies to work on a wall you share, a new wall built on the boundary, and digging foundations near a neighbour's building. Under the government's guidance you must give the neighbours notice before starting; they can agree or disagree, and a disagreement is settled under the Act, usually by surveyors. MyJobQuote advises serving notice two months to a year before work starts, and says there is nothing to pay if the neighbour agrees in writing.

Before the builders arrive you need drawings, a structural engineer's calculations, building regulations plans, party wall notices and, where you need one, a planning decision. MyJobQuote says to allow at least 8 weeks for planning approval and to start planning 3 to 6 months ahead.

Approved Document L also limits the glass. The windows, roof windows, rooflights and doors in an extension should not add up to more than 25% of its floor area, plus the area of any windows and doors the extension covers up. For a 30 m² extension that is 7.5 m² of glass before you add back the windows it replaces. If you want more, for a glazed back wall or a row of rooflights, paragraph 10.9 allows it when the whole extension loses no more heat than one built to the standard, which usually means insulating the walls and roof beyond the limits above.

If your heating is already near its limit, an extension can tip it over. Ask the builder to check the boiler or heat pump can heat the extra space, and see our heat pump running costs guide if you are weighing a heat pump at the same time. A new kitchen or shower room needs an extractor fan; our condensation guide explains why ventilation matters in a well insulated room. If you are deciding between going out and going up, our loft conversion cost guide prices the alternative the same way.

• Get the drawings and structural design done first, so every builder prices the same job.
• Ask for each line priced separately: groundworks, walls, roof, glazing, electrics, heating, plastering, any kitchen or shower room, scaffolding, waste, building control and decorating.
• Ask whether VAT is included, and who serves the party wall notices and deals with building control.
• Ask what happens if the foundations need to go deeper or a drain has to move, and hold a 10% contingency.
• Ask to see a finished extension by the same firm and speak to that homeowner.

We do not recommend extension builders yet, and no link on this page is paid. When we do, every firm will have to meet our trusted supplier standard. Until then, the Federation of Master Builders' search lists builders who belong to its trade body.

Extension prices: MyJobQuote, House Extension Cost (published and updated 19 October 2026, written by Samantha Jones, edited by Emily Dick); HomeOwners Alliance, House extension costs (undated; its rate and single storey table are credited to Checkatrade, 2025); Federation of Master Builders, Two-storey extension costs (13 January 2026); Rated People, How much does a house extension cost in 2026? (prices accurate as of 2026, including VAT); Bark, House Extension Costs 2025 (undated). All read on 11 November 2026.

Permitted development limits from the Planning Portal, extensions planning permission, which summarises Class A of the General Permitted Development Order 2015, and its mini guide to extensions, which also covers building regulations. Fees from GOV.UK, fees for planning applications in England from 1 April 2026 and fees for planning applications. Party walls from GOV.UK, preventing and resolving disputes in relation to party walls. Insulation and glazing limits from Approved Document L, Volume 1, 2021 edition incorporating 2023 amendments, paragraph 10.7 and Table 4.2, and the 2026 edition.

Heating costs from our heat model, built on the DESNZ National Energy Efficiency Data-Framework 2026, with energy prices from the Ofgem price cap for October to December 2026. Full method on our methodology and data sources pages.