Quote check
Why are builder quotes higher than our budget?
Early budgets and architects' first figures are often a price per square metre for the building work alone. Real quotes add VAT and the finishes that rough figure left out, and you still have fees and a contingency to fund. For a 25 m² single-storey rear extension that is the difference between £63,000 to £78,000 to build and £78,500 to £102,000 all in. If three detailed quotes land close together, the price is probably right and the budget was light.
Where the gap usually comes from
Most early estimates cover the first line of this table. Most people's budgets need to cover all of it.
| Stage | Typical range |
|---|---|
| Building work (what most quotes cover) | £63,000 to £78,000 |
| Survey, design, structural engineer and building control | £5,800 to £11,800 |
| Contingency of 15% | £9,500 to £11,700 |
| All-in budget | £78,500 to £102,000 |
Planning fees and a party wall surveyor come on top if your project needs them. See every fee in the hidden costs of an extension.
Check the quotes against the market
Put your size, finish and postcode into the extension cost calculator. If your quotes sit inside the range for your region, the market price is probably fair and the saving has to come from the design or the specification. If they sit well above it, ask each builder for an itemised breakdown and read why builder quotes differ.
Ways to bring the cost down
- Choose mid-range finishes. The same extension costs £51,000 to £63,000 with a budget finish and £81,000 to £99,000 with a premium one.
- Keep the shape simple. Straight walls, a simple roof and standard door and window sizes cost less to build.
- Go easy on glazing. Large sliding or bifold doors and roof lanterns are among the most expensive items per square metre.
- Leave the plumbing where it is. Moving kitchens, bathrooms and drains adds cost quickly.
- Take some work on yourself. Decorating, flooring and the garden can be done later or by you.
- Ask the builders. Ask each one where they would save money on your design. They often know a cheaper way to build the same result.
- Get more quotes on the same specification. Two or three more detailed quotes tell you whether the first ones were outliers.
- Think carefully before shrinking it. Fixed costs do not shrink with floor area, so very small extensions cost more per square metre.
| Floor area | Build cost | Per m² |
|---|---|---|
| 12 m² | £32,000 to £39,500 | £2,700 to £3,300 |
| 15 m² | £38,000 to £47,000 | £2,500 to £3,100 |
| 20 m² | £50,500 to £62,500 | £2,500 to £3,100 |
| 25 m² | £63,000 to £78,000 | £2,500 to £3,100 |
| 30 m² | £75,500 to £93,500 | £2,500 to £3,100 |
Plan the full budget
Use the renovation budget planner to set a budget that covers fees, finishes and contingency, so the next set of quotes does not come as a shock.