House Extensions London

A house extension in London is the most expensive thing most people ever add to a home, and the one where the fixed costs are least visible from the outside. If the extension is mainly a bigger kitchen, our kitchen extensions in London page goes deeper on side returns, drainage and the kitchen fit. This page is for a London homeowner deciding between a side return, a rear, a wraparound, a double storey or a basement extension, before an architect draws anything. It covers how an extension runs on site, which consents apply and who obtains them, what London build costs look like, how you live through the work and what goes wrong on other people's jobs. We write it as the contractor who digs the foundations, sets the steel and builds the shell, not as a practice that designs and hands over. The structural engineer we appoint designs the structure, we build to it, building control inspects it, and you serve the party wall notices.

Across London, a single-storey rear extension typically starts from £40,000, or £2,000 to £2,800 per m² (indicative ranges, Architecture for London, Resi, MyBuilder and Checkatrade, 2026). GS Renovation Ltd, a building contractor based in Potters Bar, does the work in London: the structural engineer we appoint designs the structure, we build it, and building control signs it off.

Which extension, for which London house: a quick decision guide

Start from the house, then the planning route, then the price line. In a Victorian or Edwardian terrace the usual first move is a side return infill beside the closet wing, then a rear or wraparound extension where the garden allows; see side return extensions in London and wraparound extensions in London. A single storey rear extension up to three metres beyond the original rear wall can be permitted development, but a wraparound goes beyond the side and the rear at once and needs a planning application. A side return runs £2,400 to £3,200 per square metre, typically £60,000 to £100,000 as a whole project, and a single storey rear extension starts from £40,000.

An interwar semi usually has a side passage and a deeper garden, so a side extension or a full width rear extension is the natural choice. A double storey side extension suits a family short of bedrooms; see double storey extensions in London. A side extension under permitted development is limited to a single storey and half the width of the original house, and two storey work normally needs a planning application. Rear build rates run £2,000 to £2,800 per square metre standard, £3,100 to £3,700 mid and £4,300+ premium.

A detached house has the most freedom: four metres of single storey rear extension can be permitted development, and up to eight metres with prior approval. Where the plot cannot take more outwards, a basement may be the answer, priced on its own engineering rather than these rates; see basement extensions in London. A flat can only extend at ground level, needs the freeholder's consent under the lease and always needs a planning application, because flats have no householder permitted development rights. On every type, add professional fees of 15 to 20% of the build cost and, where the Party Wall etc. Act 1996 applies, a single agreed surveyor at £900 to £2,700 per adjoining owner. Indicative London ranges, MyBuilder and Checkatrade, Architecture for London and Resi, 2026.

Which extension suits a London house?

The house decides the extension more than the budget does. A Victorian or Edwardian terrace has a closet wing at the back and a narrow strip of yard beside it. The side return infill is the natural first move, and our side return extension page covers it in detail. Where the garden is long enough, a rear extension across the full width adds more floor for each pound spent. Combining the two into an L shaped wraparound gives the widest kitchen diner a terrace can have, and our wraparound extension page explains when that is better value than either alone. A semi with a side passage can take a side extension where a terrace cannot, and a detached house has the most freedom of all.

Where the need is bedrooms rather than living space, a double storey extension puts a room above the new kitchen and uses the same foundations twice. What changes when there is a floor on top is set out on our double storey extension page: deeper foundations, a planning application rather than permitted development in most cases, and a closer look at the neighbour's light. A basement extension is the answer where the plot is small and the house is tall, and our basement extension page explains why it is the most engineering heavy work we do. Flats can only extend at ground level and only with the freeholder's consent. If the ground floor is the problem, start with a side return. If bedrooms are the problem, start upstairs.

How a house extension runs on site in London, stage by stage

Design comes first, and it is done by an architect or designer you appoint, not by us. The structural engineer we appoint designs the foundations, the steel and the roof structure to that layout. Planning or a lawful development certificate is obtained, building control receives a full plans application, and you serve the party wall notices. Where a shared sewer runs under the extension, the build over agreement with the water company is in place before anything is dug. Only then do we start on site. The first job is trial holes, because the engineer's foundation depth depends on what the ground is and how close the nearest tree stands. London clay shrinks and swells, and a mature tree can push foundations far deeper than expected.

Foundations are dug and inspected, drainage laid and inspected, and the walls come up as cavity construction tied into the old solid walls of the house. The rear wall is carried on a steel beam or, where the side and rear come out together at a corner, a goalpost frame, propped until the steel is packed and inspected. The roof follows, usually a warm deck flat roof with the insulation above the deck and the flashings cut into the old wall at the abutment. Glazing goes in and the shell is weathertight. Inside, first fix services, insulation, plasterboard and plaster, then second fix, floors and decoration. A new kitchen then follows as a kitchen renovation in London in its own right. Building control inspects at each stage and issues the completion certificate at the end. Your buyer's solicitor will ask for it.

Planning, permitted development and building regulations for London extensions

Many single storey rear extensions in London are permitted development, but the limits are where people go wrong. A rear extension on a terrace or semi may extend up to three metres beyond the original rear wall, four on a detached house, with the eaves and overall height capped and the materials matching the house. Larger ones, up to six and eight metres, need prior approval and a neighbour consultation first. A side extension is limited to half the width of the original house and a single storey. A flat has no householder permitted development rights. In a conservation area a single storey rear extension can still be permitted development unless an Article 4 direction removes it, but side extensions, two storey rear extensions and roof enlargements need a full planning application. A listed house needs listed building consent as well. A lawful development certificate is the proof to keep for the day you sell.

Building regulations apply whether or not planning does. Part A covers the foundations and the steel, Part L the insulation and glazing, Part B the escape routes and the wall near the boundary, and Part P the electrics. The Party Wall etc. Act 1996 applies where you excavate within three metres of a neighbour's building to a depth below their foundations, or build on the line of junction, or bear steel into a shared wall. You serve the notice as building owner, one month ahead for excavation and line of junction work and two months for work to the party wall itself. We are not party wall surveyors and do not serve notices, but we give you the drawings the notices rely on and build to the award. The notice periods are set out on our party wall page. Before any of this you need a designer, not a builder.

What does a house extension cost in London?

A single storey rear extension in London starts from £40,000, and the build rate runs £2,000 to £2,800 per square metre at a standard specification, £3,100 to £3,700 mid range, and £4,300+ for a premium finish. A side return infill runs £2,400 to £3,200 per square metre, which puts a typical one at £60,000 to £100,000 as a whole project. Professional fees add 15 to 20% of the build cost, and a single agreed party wall surveyor costs £900 to £2,700 per adjoining owner, or £1,800 to £5,400 where each side appoints their own. These are indicative London ranges, MyBuilder and Checkatrade, Architecture for London and Resi, 2026, not a quotation. VAT is charged at the standard rate on an extension to an existing house. A firm price follows a survey and the engineer's design.

The rate per square metre matters less than what the fixed costs are spread across. Every extension has foundations, drainage, steel and a roof, and on a narrow side return those costs serve very little floor, which is why the rate is highest there. What moves a quotation after that is the ground, the drain, the steel and the glazing. Deeper foundations near a tree, a sewer that has to be diverted rather than built over, a goalpost frame instead of a single beam, and structural glazing instead of a set of doors each add real money. Access adds labour when everything comes through the house. The kitchen, the flooring and the sanitaryware are on top, and they are the part of the budget you control most.

Access, neighbours and living at home during a London extension

Most London terraces have no way to the back garden except through the house, and that shapes the whole programme. Spoil from the foundations comes out in bags through the hall, and concrete goes in by pump from the street or by barrow. Steel comes through the front door or over the roof by crane on a parking suspension. We board the hall, sheet the stair and protect the floors, but for the weeks of groundworks the house is a route to a hole. Skips need a borough permit and, on a permit parking street, a suspended bay. Deliveries are timed to the hours the council allows for noisy work, and the neighbours are told the programme before the first digger arrives.

You can live in the house through most of an extension, but not comfortably through all of it. The old kitchen usually goes early, because its wall is the one coming out, so a temporary kitchen upstairs with a microwave and a sink is worth setting up. The garden is a site for the duration and the back of the house is open to the weather until the shell is closed. Scaffolding goes up on a double storey job and stays until the roof is finished. Dust travels, and the rooms you live in need doors kept shut and a screen at the stair. Most families stay put for a single storey extension and move out for a basement. We plan around either.

What goes wrong with London extensions, and how to choose a contractor

The problems we are asked to fix on other people's extensions were nearly all decided before the first brick. Foundations dug to a standard depth next to a tree that needed deeper, so the new floor cracks away from the old one in a dry summer. An extension built over a shared sewer without a build over agreement, discovered when the house is sold. A junction between the new cavity wall and the old solid wall with no insulation continuity, so a damp line appears inside every winter. A flat roof laid with no fall, ponding within a year, and an abutment flashing pointed rather than cut in. And an extension built to a permitted development assumption that was wrong, with an enforcement letter to follow.

Choosing a contractor for an extension comes down to who designs, who inspects and who is accountable. Expect a named structural engineer for the foundations and steel, a full plans application to building control with a stage by stage inspection record, and staged payments tied to those inspections rather than to the calendar. Ask whether the price includes the drainage connection, the making good where the old and new walls meet, and the flat roof detail at the abutment, because those are where the extras hide. Ask who will be on site each day. We give you one named point of contact from survey to certificate, our own trades rather than a subcontractor chain, and building control coordinated on every notifiable job. If you would rather hold one contract for the design as well as the build, our design and build page explains how that works, and our building contractor page covers the contract, the payments and the programme.

• Bi-fold, sliding and French door openings to the garden
• Levels, drainage and landscaping made good around the new build
• Full fit-out, finishing and making good

If you are weighing up options for your London property, it is worth reading RSJ and steel beam installation in London, basement extensions, a garage conversion instead and Double Storey Extension Cost.

Why London Homeowners Choose GS Renovation

London homeowners choose GS Renovation for a house extension because one team carries the job from foundations to completion certificate, and the price is set from a survey rather than a drawing. The structural engineer we appoint designs the foundations, steel and roof structure, we build to that design with our own trades, and building control is coordinated on every notifiable job. Abdelaaziz Saaid, our founder and director, brings over 30 years of hands-on experience to the survey, and a free, no obligation written quotation follows it. One named point of contact throughout. Fully insured, and rated 4.9 from 31 reviews on MyBuilder.

• Brick and mortar matched to the existing elevation
• Rated 4.9 from 31 reviews on MyBuilder
• Drainage diverted or built over with the right approval in place

Checked by Abdelaaziz Saaid, founder and director of GS Renovation Ltd (Companies House no. 14611254). Structural design is by the consulting engineer appointed to your project.