Key Takeaways
- A double storey extension in London costs £75,000 to £160,000 in 2026: about £75,000 to £100,000 for a standard project and £100,000 to £160,000 mid-range, with large or high-end projects reaching £200,000+ (LCCL Construction and MyJobQuote, 2026).
- The build rate is £1,800 to £2,600 per m² of new floor area (Resi, MyBuilder and Checkatrade, 2026). Project totals sit well above rate times area, because they include the opening into the house, the kitchen, bathroom and glazing.
- Its build rate is lower per m² than a single storey's £2,000 to £2,800 because foundations, scaffold and roof are shared across two floors.
- An extra bedroom adds around 10 to 13% to value, and a double bedroom plus a bathroom on a three-bed, one-bath house up to 24% (Nationwide, 2025).
- Most need full planning permission and take 16 to 24 weeks on site.
If you need to add serious space to a London home and have the room to build, a double storey extension delivers the most floor area for your money. This guide covers exactly what it costs in 2026, why it is so cost-efficient, and the planning and timeline you should expect.
How much does a double storey extension cost in London?
A double storey extension in London typically costs £75,000 to £160,000 in 2026: roughly £75,000 to £100,000 for a standard project and £100,000 to £160,000 mid-range, with large or high-end projects reaching £200,000 or more, according to LCCL Construction and MyJobQuote figures. The build rate on its own is £1,800 to £2,600 per m² of new floor area (Resi, MyBuilder and Checkatrade, 2026). The totals sit well above rate times area because they carry the structural opening into the existing house, the kitchen, bathroom, glazing and finishes, so budget from the totals rather than multiplying the rate. Professional fees of 15 to 20% and VAT come on top.
| Size (total, both floors) | Typical London cost |
|---|---|
| Standard (20 to 30 m²) | £75,000 to £100,000 |
| Mid-range (30 to 45 m²) | £100,000 to £160,000 |
| Large / high-spec (45 m²+) | £160,000 to £200,000+ |
Costs by size and specification
As with any build, the final figure depends on size, specification and site conditions. A compact two-storey rear extension on a terraced house sits at the lower end, while a larger extension with high-end finishes, structural complexity, or difficult access in a tight London plot pushes towards and beyond £160,000. Restricted access, scaffold and parking permits, and the need to work around neighbours all add to London costs.
Why a double storey is cost-efficient
On build rate, a double storey extension is cheaper per square metre than a single storey: £1,800 to £2,600 per m² against £2,000 to £2,800 for a single storey (Resi, MyBuilder, Checkatrade and MyJobQuote, 2026). That is because the most expensive elements, the foundations, scaffolding and roof, are shared across two floors rather than paid for once per floor. You are essentially getting the second floor at a discount, because the groundworks and roof are already there. For homeowners who know they need a lot of extra space, building up as well as out is the most efficient way to get it.
What to do with the space
The classic configuration adds a larger kitchen-diner or living space downstairs and an extra bedroom upstairs, very often a master suite with an en-suite and dressing area. Adding a bedroom and a second bathroom is exactly what buyers value most, which is why the double storey is such a strong investment. Some homeowners use the upstairs for a guest suite, a home office, or a children's bedroom and bathroom.
Planning permission
Most double storey extensions exceed Permitted Development limits and need full planning permission, particularly on height, depth and impact on neighbours' light and privacy. In a conservation area a two-storey extension always needs planning permission, because designation removes it from permitted development, and an Article 4 direction can add further controls. The good news is that English councils approve around 90% of householder applications (gov.uk planning statistics), and a well-designed scheme that respects the neighbours has a strong chance. You will also need a party wall award where the work affects shared walls or excavates near a neighbour's foundations, which is common in London terraces. You serve the notices as building owner, or your surveyor does; we give you the drawings and programme they need and build to the award.
Timeline and value
A double storey extension typically takes 16 to 24 weeks on site, with design, planning and structural calculations beforehand. Against that, the value uplift is among the best available: Nationwide's 2025 research puts an extra bedroom at around 10 to 13%, rising to up to 24% where a three-bed, one-bath house gains a double bedroom and a bathroom, the combination a double storey is built to deliver.
GS Renovation designs and builds double storey extensions across all of London. We manage the planning application and the build; the structural engineer we appoint designs the steel and foundations, and building control inspects and signs off the work. For a free assessment of what your property can support, contact us or call 07472 424 226. Comparing options? See our extension cost guide.