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 on this page are drawn from genuine jobs so you can compare properly rather than guess.
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 that a single storey box extension would not need.
- 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 because of its height and its effect on neighbouring windows and boundaries.
- If it sits on a shared boundary wall, a party wall agreement is often needed before work can start, and sorting this early avoids a stalled site later.
- Structural work, especially steel beams to open up the ground floor, is one of the biggest swings between a cheap quote and an expensive one for what looks like the same size extension.
- Two quotes for the same footprint can differ a lot once you check whether scaffolding, building control fees and steelwork are actually included, rather than added later as extras.
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, and the size and number of beams needed depends on the span and the load above them. Foundations depend on the ground and any nearby trees, so a site with clay soil or roots close by can need deeper or more complex foundations than a straightforward one. Matching the existing brick, render or roof tiles can add cost if the originals are hard to source, especially on an older house where the brick was never a standard type. Access and scaffolding for two full storeys also feed in, since a tight side return or a terrace with no rear access makes everything slower and more expensive than a detached house with room to work. Who you use matters too. A small local builder working directly with you tends to carry less overhead than a larger firm with project managers and a sales layer, and that difference shows up in the quote even when the finished job looks the same. Labour and material costs also differ by region, which is another reason the same size extension can come in at very different prices depending on where in the country you are, so the real prices on this page are worth comparing against others near you rather than a national one.
How to pay less
- Stack bathrooms and the kitchen above and below each other so plumbing and drainage runs stay short and simple, cutting both material and labour time.
- Match the design to your existing roof line and brick to avoid expensive bespoke materials and tricky detailing where new meets old.
- Get three detailed quotes and make sure each includes steelwork, scaffolding and building control, not just the brickwork, so you are comparing like with like.
- Do the messy first-fix stage in one go rather than phasing the work, which usually costs more in repeat mobilisation, skip hire and scaffolding hire.
- Keep the footprint simple. Extra corners, dormers or changes in roof height each add labour and materials that a straightforward rectangular box avoids.
- Ask your builder about timing. A quiet period in their diary can mean a keener price than booking work for their busiest months.
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 on this page 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.
Why do quotes for the same size extension vary so much?
Two quotes that look similar on paper often cover different things. One builder may include scaffolding, steelwork and building control fees, while another prices those as extras once the job starts. Ground conditions, access, and how closely the new brick or roof needs to match the old also push the figure up or down, so always ask what is actually included before comparing numbers.
Does the extra bathroom upstairs really add that much?
Yes, more than most people expect. It is not just the fittings. New waste and supply pipes have to run down through the building to connect to existing drainage, the floor needs waterproofing, and ventilation has to be sorted properly. That work sits behind the walls where you cannot see it, but it still has to be paid for.
Should I phase the work to spread the cost?
It rarely saves money overall. Splitting a two storey extension into stages usually means paying twice for scaffolding, site setup and skip hire, and a builder has to remobilise a crew that has moved on to another job. If cash flow is the issue, it is usually cheaper to agree a staged payment schedule with one continuous build than to physically break the work into two separate projects.