Cost advice for your building project

Plan with a quantity surveyor's eye

Most homeowners build once or twice in their lives. Builders price work every week. John has worked in London construction since 1983, and this page puts that experience on your side of the table.

Whether you are planning an extension, a loft conversion or a garden room, the same three questions come up: what will it really cost, what have I missed, and is my builder's quote fair? Start here.

Budget tools

7 costs most extension quotes leave out

When a quote looks cheap, it is usually because something is missing. Check yours for these:

1. Party wall surveyor fees, if you are building on or near a shared wall.
2. Building control fees, charged by the council or an approved inspector.
3. Structural engineer's design and calculations for beams, foundations and openings.
4. Moving drains that run under the new footprint.
5. Skips and muck-away, which add up quickly on a dig for foundations.
6. Making good and decorating where the new work meets the old house.
7. VAT, if the quote does not say clearly whether it is included.

How much contingency should you allow?

Add 10 to 15% contingency on top of your build cost. Aim for the top of that range on older houses, or where nobody yet knows what the ground is like.

Provisional sums: why your quote can grow

A provisional sum is an allowance for work nobody has fully designed or priced yet, such as foundations or the kitchen fit-out. The real cost replaces the allowance later, so a low figure makes a quote look cheaper than it really is.

Ask your builder which items are provisional, how each figure was reached, and how the final cost will be agreed before that work starts.

Have these ready before you ask for quotes

Builders can only price what they can see. Give every builder the same information and you get quotes you can actually compare.

• Full architect's drawings
• Structural engineer's calculations
• Planning permission, or confirmation the work is permitted development
• A written list of finishes: flooring, kitchen, sanitaryware, glazing
• Your realistic budget ceiling

Which loft conversion suits your house?

• Rooflight: the least disruption; the roof shape stays the same.
• Rear dormer: a big gain in headroom, and common on terraced houses.
• Hip to gable: for semis and detached houses with a hipped roof.
• Mansard: the most space, but usually needs planning permission.

Garden room base: concrete slab or ground screws?

A concrete slab means digging out, carting away spoil and waiting for it to cure, but it is permanent and handles heavy, masonry-style buildings.

Ground screws need little or no excavation, can often be installed in a day and can be removed later. They need a ground test first to confirm the soil will hold them.

Got a quote already?

If you have a builder's quote in hand, have it checked before you sign. We review every line against current rates and tell you what is missing, what is at risk and what a fair price looks like.

Find out about our builder's quote check →

Looking for tools? See the kit we recommend.