Rear Extensions in London: Cost, Planning and Design

A rear extension builds out from the back wall of the house into the garden. On a London terrace or semi it is the usual way to turn a separate kitchen and dining room into one open room for cooking, eating and living, with the garden on the other side of the glass.

This guide covers the three things that decide a rear extension, as of October 2026: what it costs, what planning allows, and the design choices that make the room worth having. For the choice between a rear extension and a side return, see our rear extension or side return guide.

• Single storey: the most common. It adds depth to the ground floor, usually for an open plan kitchen, dining and living room, and it is the one most often built under permitted development.
• Two storey: a room on each floor in one build, such as a larger kitchen below and a bedroom and shower room above. Its permitted development limits are tighter, and in London it often needs a planning application.
• Wrap around: a rear extension combined with a side return infill in an L, for a room across the full width of the house. It is the most transformative and usually needs a planning application.

From typical London jobs priced on our estimator in October 2026, all ex VAT and indicative rather than a quotation:

• A single storey rear extension of about 20 m² of new floor, flat roofed with no rooflights, plus making good the room it joins: around £57,000, roughly £2,900 per square metre of new floor.
• New above ground space at a standard specification: £2,650 to £2,900 per square metre of new floor.
• A two storey side return adding a kitchen extension with a bedroom and shower room above, about 48 m² of new floor: around £128,000.

Our minimum for a complete extension project, once finishes and fittings are included, is around £85,000. The figures price windows, doors, any rooflights, sanitaryware and tiles for fitting only, with the products supplied by you, and leave out kitchen units, worktops and appliances. The full breakdown is in our house extension cost guide, and the extension cost calculator prices your own in about two minutes.

A single storey rear extension to a house can often be built without a planning application, within these limits in England as of October 2026:

• Up to 3 metres beyond the original rear wall of a terraced or semi detached house, or 4 metres for a detached house.
• Up to 6 metres, or 8 metres for a detached house, through the prior approval route, in which the council consults the neighbours before deciding.
• No more than 4 metres high, and within 2 metres of a boundary the eaves may be no higher than 3 metres.
• Together with other additions, it may not cover more than half the land around the original house, and the materials should look similar to the existing house.

The prior approval route is not available in conservation areas, flats and maisonettes have no permitted development rights, and a listed building needs listed building consent. Where you rely on permitted development, a Lawful Development Certificate from the council confirms it in writing. Our step by step guide to planning a house extension covers the full sequence.

Most London rear extensions involve the Party Wall etc. Act 1996:

• New foundations within 3 metres of the neighbour's building, and deeper than its foundations, need one month's excavation notice.
• A new wall at the line of junction with the neighbour needs one month's notice.
• Work to an existing party wall, such as cutting in a beam, needs two months' notice.

Drains are the other hidden constraint. Building over or close to a public sewer needs the water company's agreement, so we survey the drains before the design is fixed rather than after.

• Light: a deep rear extension can leave the middle of the house darker. Rooflights or a glazed roof near the old back wall bring daylight to the centre of the plan.
• The opening: joining the extension to the house means taking out part of the back wall on new steel beams, which is what makes the room feel like one space rather than two.
• The garden side: wide glazed doors connect the room to the garden, and a level threshold makes the two feel continuous.
• The roof: a flat roof keeps the height down near the neighbours, while a pitched or partly glazed roof gives more volume where planning allows.

At St Germans Place in Blackheath, SE3, we designed and built a rear extension that created additional space for the kitchen, with Velux skylights and traditional style wooden doors opening onto a new patio: see the St Germans Place project.

At Milford Mews in Streatham, SW16, a double storey extension created an open plan living room lit from two aspects, with a principal bedroom suite above, finished outside in brick and timber cladding: see the Milford Mews project.

MVV designs and builds rear extensions across West, South West, North West, Central and South East London, from our office in Chiswick, W4.

Start with the house, not the drawing: a survey of the structure, the drains and the planning position shows what is possible before anyone commits. We then sketch the options side by side, so the trade offs between depth, light and cost are visible. For a first budget, the extension cost calculator gives an indicative London range in about two minutes.