Double Storey Extension London

A double storey extension, also called a two storey extension, adds to the back or the side of a house on both the ground and first floors at once. In London that usually means a bigger kitchen and dining space below and a bedroom, or a bedroom and a bathroom, above. It is one contract, one scaffold, one set of foundations and one programme, which is the whole reason it is worth considering rather than doing the ground floor now and the upstairs in five years.

Most people arrive at this decision having priced a single storey rear extension and found that it solves the wrong problem. A wider kitchen does not help a family that has run out of bedrooms. Building up as well as out is what turns a three bedroom house into a four bedroom house without moving, and in most of London the stamp duty and agents' fees on moving would have paid for a large part of it.

The economics of building up as well as out

The expensive parts of any extension are the parts you only pay for once. Foundations, drainage diversions, scaffolding, the roof, site set up, welfare and the professional fees are all largely fixed whether you build one storey or two. Add a first floor and those costs are spread across roughly double the floor area, which is why the first floor of a two storey extension almost always costs less per square metre than the ground floor beneath it.

That does not make it cheap. A double storey rear extension in London starts around £75,000 and rises quickly with size and specification. What it makes it is better value per square metre than the same two projects done separately, and considerably less disruptive: one period of living on a building site rather than two.

Scope	What it covers	Typical London cost
Double storey rear extension, from	Standard finish, modest footprint, one bedroom above a widened kitchen or dining space	from £75,000
Double storey rear, mid-range	Larger footprint, en suite above, structural opening into the existing house, good glazing	£100,000 to £160,000
Per m², ground floor	New build floor area, standard to mid-range finish	£2,000 to £2,800 per m²
Per m², mid-range finish	New build floor area with better glazing, underfloor heating and joinery	£3,100 to £3,700 per m²
En suite added above	Second bathroom formed within the new first floor	£8,000 to £15,000
Professional fees	Architect or designer, structural engineer, planning and building control	10 to 15 per cent of build cost
Contingency	Foundations, drainage and existing structure once opened up	10 to 15 per cent

Indicative London ranges, 2026. Trade and project costs from MyBuilder and Checkatrade; per square metre build rates from Architecture for London and Resi.

Comparing that against the alternatives is worth doing properly rather than by instinct. Our double storey extension cost guide breaks the figures down line by line, and the extension versus loft conversion comparison is the right place to start if the bedroom is the whole point, because a loft conversion sometimes delivers it for less.

Planning permission is more likely than you think

This is where two storey extensions differ most sharply from single storey ones. Permitted development does allow rear extensions of more than one storey, but the conditions are tight: no more than 3 metres beyond the original rear wall, no closer than 7 metres to the rear boundary opposite, a roof pitch matching the existing house as far as practicable, upper floor side windows obscure glazed and non opening below 1.7 metres, and materials of similar appearance. On a typical London plot the 7 metre rule alone rules most of it out.

Add to that the fact that two storey side extensions are not permitted development at all, that nothing beyond a side elevation is permitted on article 2(3) land such as a conservation area, and that flats have no rights, and the honest position is that most double storey extensions in London need a full planning application. That is not a problem, it is a programme item. Budget eight to thirteen weeks for a householder application to be determined, and design the scheme knowing it will be assessed on daylight to neighbours, overlooking and the impact on the rear building line.

We are the contractor, not the architect and not the planning consultant. On a project like this we either work to your appointed designer's drawings or coordinate the appointment for you, and we flag buildability problems while they are still lines on paper. Our London planning permission guide explains the process and what actually gets refused.

What changes structurally when you go up

A second storey changes the engineering, not just the quantity. The loads travelling down are roughly double, which affects the foundations, the wall construction and every opening you form into the existing house.

• Foundations sized for two storeys, and taken deeper where trees, clay soil or a neighbour's deeper footings require it, all specified by the engineer and inspected before concrete
• Steel beams at both levels where openings are formed into the existing house, each with its own calculations and padstones
• Tying the new structure into the old, including how the new first floor bears and how the existing wall is restrained during the work
• The existing roof cut back and the new roof tied in, with the junction detailed to keep water out for the next fifty years
• Upstairs layout consequences: the landing, the stair, where the existing bathroom goes and how the new rooms are reached
• Part B means an escape route from the first floor, and Part L means the new thermal elements meet current standards rather than matching the old house
• Building control inspections at foundation, damp proof course, steel, drainage, insulation and completion

Where a public sewer runs near or under the footprint, a build over agreement is needed from the water authority before work starts. Where the excavation goes within 3 metres of a neighbour's foundations and deeper than them, or the work cuts into a party wall, the Party Wall etc. Act 1996 applies and notice must be served by you as the building owner. We prepare the ground for both and programme around them.

Living through a two storey build

A single storey extension leaves the upstairs of your house alone. A two storey one does not. There will be a period with scaffolding across the back of the house, a bedroom window boarded, and a week or two where the rear wall is open at first floor level and the building is sheeted rather than sealed. Most clients stay in the house, and it is manageable, but it is worth planning for rather than discovering.

The way we reduce it is sequencing: get the shell up, the roof on and the building watertight before anything is opened into the existing house, so the disruptive phase is short and predictable. Our order of works guide sets out why that sequence matters, and the guide to living in your home during a renovation is honest about the rest.