How much does a two storey extension cost in London in 2026?

A two storey extension, sometimes called a double storey extension, adds a room on the ground floor and another above it in one build. On a typical London job priced on our estimator in September 2026, a two storey side return adding a kitchen extension with a bedroom and shower room above, about 48 m² of new floor, came to around £151,000 ex VAT. That is roughly £3,150 per square metre of new floor, at the lower end of the £3,100 to £3,400 per square metre we see for new extension space at a standard specification, because the upper floor shares its foundations and roof with the room below.

The figures here are indicative, not a quotation. They come from the same engine that powers the instant estimator on this site, exclude VAT, and come before professional fees, planning and building control. Here is what drives the number, and what the planning rules allow.

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

• A two storey side return adding a kitchen extension with a bedroom and shower room above, about 48 m² of new floor: around £151,000, roughly £3,150 per square metre of new floor.
• For comparison, 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 £67,000, roughly £3,400 per square metre.
• New above ground extension space at a standard specification: about £3,100 to £3,400 per square metre of new floor.

Our minimum for a complete extension project, once finishes and fittings are included, is around £100,000. In the job figures, windows, doors, any rooflights, sanitaryware and tiles are priced for fitting only, with the products supplied by you, and kitchen units, worktops and appliances are not included. VAT at 20% comes on top, which takes £151,000 to £181,200. Our house extension cost guide sets these figures beside single storey extensions, refurbishments and basements.

Two of the biggest costs in any extension sit at the bottom and the top: the foundations and ground floor slab, and the roof. A single storey extension pays for all of them to create one floor. A two storey extension pays for them once and gets two floors, so each square metre of the upper floor carries less of their cost. That is why our two storey example comes in at roughly £3,150 per square metre of new floor, against about £3,400 for the single storey one.

Three things push the other way:

• Wet rooms upstairs: a bathroom or shower room on the new first floor concentrates plumbing, waterproofing and tiling in a small space.
• Joining the house at two levels: opening the existing back or side wall on both floors needs more steel and more temporary support than opening it on one.
• Height and access: scaffolding and working at first floor level, often through a narrow side passage, cost more than building at ground level.

A lower price per square metre is not a lower total. A two storey extension usually costs more overall because it builds more floor: our two storey example adds about 48 m², the single storey one about 20 m². When you need the space, that is exactly where the value lies.

Usually, yes. Houses in England have permitted development rights, set out in Class A of the General Permitted Development Order, but they are much tighter for anything taller than one storey. As of September 2026, a rear extension of more than one storey is permitted development only if:

• it extends no more than 3 metres beyond the rear wall of the original house, whether the house is terraced, semi detached or detached,
• it stays at least 7 metres from the boundary opposite the rear wall, and
• within 2 metres of any boundary, its eaves are no higher than 3 metres.

The eaves of a second storey are almost always higher than 3 metres, so in practice a two storey extension has to stand more than 2 metres clear of every boundary to qualify, which rules it out on most London terraces. Where it does qualify, the materials must look similar to the house, the roof pitch must match the original as far as practicable, and any upper floor window in a side wall must be obscure glazed and fixed shut unless the part that opens is more than 1.7 metres above the floor.

Several cases always need a planning application:

• Any side extension of more than one storey.
• A rear extension of more than one storey in a conservation area.
• Flats and maisonettes, which have no householder permitted development rights.
• Houses where an Article 4 direction has removed the rights.

The larger rear extension route, up to 6 or 8 metres through prior approval, is for single storey extensions only, and across the plot, extensions and outbuildings may not cover more than half the land around the original house.

So on most London houses a two storey extension means a householder planning application. The fee in England has been £548 since 1 April 2026, with a rise to £575 proposed from December 2026, and the council has eight weeks from receiving a valid application to decide it, unless you agree to more time. Councils judge a two storey extension largely on its effect on the neighbours' light and outlook. Lambeth's design guide, for example, checks an extension against the neighbour's nearest window with a 45 degree test on plan and in elevation, and Brent's 2025 extensions guide stops a two storey rear extension at a 45 degree line taken from the middle of the nearest neighbouring habitable room window. Our guide to planning a house extension puts the steps in order.

A two storey extension on a London terrace or semi usually involves the Party Wall etc. Act 1996:

• A new wall built at the line of junction with the neighbour needs at least one month's notice.
• Work to an existing party wall, such as cutting it for steel beams or raising it, needs at least two months' notice.
• Digging foundations within 3 metres of the neighbour's building and deeper than its foundations, or within 6 metres where the dig would cut a 45 degree line down from them, needs at least one month's notice, with plans and sections.

Drains come next. Thames Water, which serves most of London, says a build over agreement is likely to be needed for work within 3 metres of a public sewer or 1 metre of a public lateral drain. Planning permission does not cover it, and without one the building regulations completion certificate may be withheld. A camera survey of the drains early in design turns this from a surprise into a line on the drawing.

Every extension needs building regulations approval, covering structure, fire safety, insulation and ventilation, and a two storey extension brings two fire safety requirements with it. A new bedroom on an upper floor served by a single staircase needs either direct access to a protected stair or an emergency escape window, with an opening of at least 0.33 m², at least 450 mm high and 450 mm wide, and its bottom edge no more than 1,100 mm above the floor. And adding a habitable room above the ground floor means providing a fire detection and alarm system for the house.

Four costs sit outside the build figures above:

• VAT: building an extension to an existing house is standard rated, so VAT at 20% applies to the building work. Lower rates apply only in narrow cases, such as renovating a home that nobody has lived in for the two years before work starts, or where the extension itself creates a new self contained home. Extensions to listed houses are standard rated too.
• Community Infrastructure Levy: an extension adding less than 100 m² of gross internal floor area that does not create a new home is exempt, with nothing to claim. At 100 m² or more, a homeowner extending the house they live in can still be exempt, but only if the claim reaches the council before work starts, and the claim lapses if work starts before the council has decided it.
• Planning fee: £548 for a householder application in England since 1 April 2026.
• Professional fees: design, structural engineering, party wall surveyors and building control all come on top of the build cost.

A second storey earns its cost when the brief needs a bedroom or a bathroom as well as a bigger ground floor, when the garden is too short to give up more of it, and when the planning position supports building up. It adds floor area without taking any more garden than the ground floor already does. If the space you are missing is only a bedroom and a bathroom, a loft conversion is the other route to compare: our loft conversion cost guide has the figures, and our guide to the rear extension or side return choice covers the ground floor options.

Every two storey extension is priced from the house in front of us: the survey, the drains, the planning position and the brief. At Milford Mews in Streatham, SW16, we designed and built a double storey extension with an open plan living room on the ground floor and a principal bedroom suite above, housing a private sauna, a bespoke fitted wardrobe and a home office, finished in brick and timber cladding. You can see the project. To see what your own extension would cost, the instant estimator prices it from your description in minutes, or talk to us.