What will itcost?
Choose the project, set the size and pick a finish. The range includes VAT, building control, party wall and design fees, so the number is closer to what you will really pay.
Six things that move the number.
Size is only the first one. These are the others, with honest ranges.
Ground and foundations
Clay, tree roots and a high water table can add trench-fill concrete or piles. We dig a trial hole at survey, so it is in the price, not found on day three.
Typical: £2,000 to £12,000 extraStructure and steel
Opening up the back wall needs a steel beam and padstones. A wider opening or a loft stair needs bigger steels and more temporary support.
Typical: £2,500 to £9,000Glazing
Bifolds, slim-frame sliders and a roof lantern are the biggest single choices after size. Triple glazing and solar-control glass add more.
Typical: £9,000 to £23,000Kitchen and fit-out
A kitchen at £18,000 and one at £45,000 are both reasonable. We separate it from the build so you can see what the room itself costs.
Typical: £18,000 to £45,000Neighbours and access
A terrace with a shared wall costs more than a detached house: party wall awards, a longer scaffold and sometimes a crane for steels.
Typical: £1,000 to £2,000 per neighbourPlanning route
Permitted development with a lawful development certificate is quicker and cheaper. If your house is in a conservation area, or the design goes beyond limits, it needs planning.
Council fee: £129 or £258The rates behind the estimator.
Building work only, including VAT. Design, engineering and fees are listed separately in the estimate so nothing is hidden.
| Type | Essential | Considered | Architectural | Size range |
|---|---|---|---|---|
| Single-storey rear | £2,300 | £2,900 | £3,800 | 8 to 45 m² |
| Side return | £2,500 | £3,150 | £4,100 | 8 to 24 m² |
| Wraparound | £2,450 | £3,050 | £3,950 | 30 to 60 m² |
| Double-storey | £3,680 | £4,640 | £6,080 | 16 to 45 m² |
| Type | Essential | Considered | Architectural | Typical size |
|---|---|---|---|---|
| Loft dormer | £55,000 | £68,000 | £85,000 | 30 m² typical |
| Hip-to-gable loft | £62,000 | £76,000 | £95,000 | 32 m² typical |
| Mansard loft | £78,000 | £96,000 | £120,000 | 32 m² typical |
What people ask about cost.
Is this a quote?
No. It is a range built from our own recent jobs. After the design visit and survey we write a fixed price, and that is the number in your contract. If a range and a price ever differ by a lot, we tell you why.
Why is a double-storey extension not double the price?
You share one set of foundations, one roof and one set of drains between two floors. We count it at about 1.6 times the single-storey cost for the same footprint.
Does the range include VAT?
Yes. Extensions and loft conversions on an existing home carry 20% VAT, and every figure on this page already includes it. Council fees are not subject to VAT.
What does a party wall surveyor cost?
Between £1,000 and £2,000 for each neighbour who is served notice, if they appoint a surveyor, which we recommend you pay for. You can choose the same surveyor as your neighbour to save cost.
When do I pay?
In stages tied to finished work. The first payment is no more than 10%, and the last 5% is held until the snagging list is cleared. You can see the schedule on the process page.
What if the ground is worse than expected?
We dig a trial hole at the survey stage and our engineer designs foundations from what we find. Anything unforeseen below that is named as a capped provisional sum in the contract, never an open-ended extra.
Turn a range into a price.
Book a free design visit. We measure, ask what you need and come back with a written range for your house, not an average.
Book a free design visit Fixed price within 3 weeks of the survey