How much does a rear extension cost in London in 2026?

The number of storeys decides most of the price before any choice of finish. A single-storey extension adds ground floor space, usually a bigger kitchen and living room opening onto the garden. A double-storey extension adds a room above as well, often a bedroom or a bathroom. The second storey costs less per square metre, because the foundations and roof are shared, but more in total.

The table gives the all-in total for each at three finishes on a typical house, with the floor area assumed. Each range is wide on purpose. The low end is a plain extension on an easy site. The high end is a harder site with more to put right.

Take a two-storey mid-terrace house with a single-storey extension 3.5 m deep and 5 m wide. That adds about 17.5 m² of floor. With a good-quality finish, the building work is £49,000 to £82,000. Add the fees and the contingency and the all-in total is £61,000 to £114,000, with a likely figure of about £88,000.

The likely figure is the middle of the range, not a promise. Your own house may land anywhere in it, or outside it, once the ground, the drains and the steel are known.

Measured on the building work alone, that single-storey extension works out at about £3,750 per m² of new floor, in a range of £2,800 to £4,700. A double-storey extension is about £3,100 per m², because two floors share one set of foundations and one roof.

Per square metre is a rough guide for a small extension. Much of the cost is fixed whatever the size: the steel, the drains, the opening into the house and building control. A small extension spreads those over fewer square metres, so its rate looks high. Budget from the totals and use per m² only to compare quotes for extensions of a similar size.

Two things move the price of a rear extension more than its size: the steel that opens up the back of the house, and the glass that goes in its place. Taking out the back wall means new steel beams to carry the floors and walls above. Most terraces also need a steel frame where the side wall comes out. Each beam needs a structural engineer's calculation, and a long span or a crane to lift it adds cost.

The glazing is the other big choice. Adding bifold or sliding doors across the back adds about £500 to the likely figure at a good-quality finish. A wide picture window, a roof lantern or frameless glass adds more again, and slim frames need stiffer steel above them.

Depth matters too, because every extra metre means more foundation, roof and wall. Going one metre deeper adds about £5,000; one metre shallower saves about £21,000. A new kitchen adds about £14,000, and it is often the line people forget to budget.

The building work covers groundworks and foundations, drainage, the structure, the roof, the glazing, and the first and second fix of electrics, plumbing, plaster and joinery, ready to decorate. The totals then add the fees for a single-storey extension: architect or designer £4,000 to £10,500; structural engineer £1,000 to £2,500; council prior approval fee £334 with the Planning Portal charge; building control £1,000 to £2,000; party wall surveyors £0 to £6,000; Thames Water build-over agreement £0 to £1,000. Together the fees come to £6,500 to £22,500. The party wall line is zero if the neighbours consent in writing. The build-over line is zero if no public sewer runs near the new foundations.

On top of that sits a contingency of 12% of the build, about £6,000 to £10,000 here. It is money to have available for what is found once the ground is open, not a cost you will certainly pay.

Outside the totals: a new kitchen, flooring, decorating, furniture and blinds, landscaping the garden that is left, and any time living elsewhere while the work is done. The council fees are in the box below.

Build costs move with parking, access and labour. On the London baseline a single-storey rear extension with a good-quality finish is £61,000 to £114,000. In Barking and Dagenham, Bexley and Havering, the lowest-cost end of the scale, it is £57,000 to £107,000. In Kensington and Chelsea and Westminster, the highest, it is £70,000 to £130,000.

The borough changes the building work, not the statutory fees or the planning rules. Each borough's page, linked below, gives the totals for every extension type there.

Depth decides the planning route as well as the price. On a terraced or semi-detached house, a single-storey rear extension up to 3 m deep is normally permitted development, subject to the limits checked in the report. Assuming the usual 3.5 m depth, the size priced here, it is beyond the standard 3 m but within the 6 m larger home extension allowance. That route needs prior approval from the council, with neighbours consulted, and is not available in a conservation area. At 3 m or less, no prior approval is needed. So the route for the extension priced here is: larger home extension: prior approval needed. A conservation area or an Article 4 direction changes that street by street, so check with the council. The guide "Do I need planning permission for a rear extension?", linked below, sets out every limit with its source.

House extension costs, the planning route and a timeline in each London borough.

Barking and Dagenham · Barnet · Bexley · Brent · Bromley · Camden · Croydon · Ealing · Enfield · Greenwich · Hackney · Hammersmith and Fulham · Haringey · Harrow · Havering · Hillingdon · Hounslow · Islington · Kensington and Chelsea · Kingston upon Thames · Lambeth · Lewisham · Merton · Newham · Redbridge · Richmond upon Thames · Southwark · Sutton · Tower Hamlets · Waltham Forest · Wandsworth · Westminster

Is a rear extension cheaper than a loft conversion?

Not usually per square metre. A single-storey rear extension of about 17.5 m² is £61,000 to £114,000 all in at a good-quality finish, about £3,750 per m² for the building work. A rear dormer loft of about 28 m² is £63,000 to £116,000, about £2,400 per m². The extension needs foundations, drainage, steel and a lot of glass. The loft reuses the walls already there.

How long does a rear extension take?

About 23 to 43 weeks from first sketch to sign-off for a single-storey rear extension, of which 8 to 16 weeks are on site. Design, the council's decision, the party wall notices and the building regulations drawings take most of the rest.

Do these rear extension prices include VAT and fees?

Yes. Every figure includes VAT at 20%, and the all-in totals include the professional and council fees, £6,500 to £22,500 for a single-storey extension, and a contingency of 12% of the building work at a good-quality finish (10% at straightforward, 15% at high end). A contractor's quote is usually for the building work only, so compare it with £49,000 to £82,000.