DATA-BACKED GUIDE · UPDATED JULY 2026
Two storey extension cost UK: what a double extension really costs
A two storey extension gives you space downstairs and an extra bedroom or bathroom above, all off one set of foundations and one roof. Because those big fixed costs are shared across two floors, a double extension often works out better value per room than a single one. The real prices below are drawn from genuine prices so you can compare properly.
The quick version
- You pay for one set of foundations and one roof but get two floors, so the cost per room is usually lower than a single storey job.
- Adding a bathroom or ensuite upstairs raises the price through extra plumbing, waterproofing and drainage.
- A two storey extension may still fall under permitted development within the limits, though it is more likely to need a planning check than a single storey one.
- If it sits on a shared boundary wall, a party wall agreement is often needed before work can start.
What people actually paid
Real prices, in people's own words
- £45,000“Single storey 6.5m wide x 4m long (2022) peak inflation material costs. Included 4 keylite windows and 8ft bi fold. Gable roof. £45k”
- £51,816“Total cost £51,816 (give or take a couple of quid)”
- £80,000“We just finished a single storey. It was £2.5k per square meter plus VAT. Cost 80k in total”
- £95,000“Approx 95k, 5x3m. Included a new kitchen, downstairs WC, exterior power, knocking together upstairs bathroom & WC to make 1 room and unexpected new electrical works to most of the downstairs.”
- £95,000“Total cost £95k, of which build cost was £65k. Kitchen & utility £15k, flooring £3k, architect and planning £2k”
- £100,000“Our recent 2 storey side extension was upwards of £100k building costs.”
Genuine amounts posted publicly. We publish the price and the quote, never the person.
Why the price varies so much
Size and layout set the baseline, but the finishes decide the final figure. A ground floor open-plan kitchen with a new bathroom above costs more than two plain rooms because of the fit-out. Structural work is a factor, since removing internal walls to open up the ground floor means steel beams. Foundations depend on the ground and any nearby trees, and matching the existing brick, render or roof tiles can add cost if the originals are hard to source. Access and scaffolding for two full storeys also feed in, which is why quotes on the same size vary so widely.
How to pay less
- Stack bathrooms and the kitchen above and below each other so plumbing and drainage runs stay short and simple.
- Match the design to your existing roof line and brick to avoid expensive bespoke materials and tricky detailing.
- Get three detailed quotes and make sure each includes steelwork, scaffolding and building control, not just the brickwork.
- Do the messy first-fix stage in one go rather than phasing the work, which usually costs more in repeat mobilisation.
Common questions
Is a two storey extension better value than a single?
Often yes, per room. The expensive parts, foundations and roof, are shared across both floors, so the second storey adds space at a lower marginal cost. The prices below let you compare a single and double extension side by side for your area.
Do I need planning permission for a two storey extension?
A two storey extension can still fall under permitted development if it stays within the size and height limits and respects boundary distances, but it is more likely to need checking than a single storey one. Building regulations sign-off is always required and is separate from planning.
What is a party wall agreement and do I need one?
It is a legal agreement with a neighbour when your work affects a shared wall or foundation near the boundary. For a two storey extension on a shared wall it is often needed, so serve notice early to avoid delays.