You asked three builders to price the same extension. One says £74,000, another £91,000, the third hasn't come back at all. Nothing about your project changed between those visits, so what happened?
Almost always, the answer is that they didn't price the same job.
They were never quoting for the same work
A set of planning drawings shows the shape of your extension: where the walls go, how it looks from the garden, how big the rooms are. That is what the council needs to make a decision.
It is not enough information to build from, and it is nowhere near enough to price accurately.
Planning drawings usually don't state:
- what the kitchen costs, or whether it's included at all
- which windows and doors, in what material, with what glazing
- what the floor finish is
- how the extension is heated, and whether the boiler copes
- where sockets, switches and lights go
- what happens to the garden after the digger leaves
- who pays for skips, scaffolding, welfare and site protection
Each builder fills those gaps with their own assumptions. One assumes a £4,000 kitchen, another assumes you're supplying it. Neither is being dishonest, and the two quotes are not comparable.
The three things that create most of the gap
Provisional sums
A provisional sum is a placeholder: a number inserted because the work isn't defined yet. "Allow £3,000 for glazing" is a provisional sum. If the doors you eventually choose cost £9,000, the price goes up by £6,000 and the builder is entitled to charge it.
A cheap-looking quote often contains more provisional sums than an expensive one.
Exclusions
Read the bottom of the quote. Structural steel, drainage alterations, building control fees, party wall costs and making good the garden are commonly excluded. Excluded work doesn't disappear — you simply pay for it later.
Preliminaries
Scaffolding, skips, site set-up, welfare facilities, insurance and the site manager's time. Some builders itemise these, some bury them in the rates, and some leave them out entirely and add them once work starts.
Why the cheapest quote is often the most expensive
A builder who has priced thoroughly looks expensive next to one who has priced optimistically. Once construction starts and the missing items surface as variations, you have no competitive pressure left: the scaffold is up, the wall is down, and changing builder is not realistic.
The lowest number on day one and the final invoice are frequently not the same ranking.
How to get quotes you can actually compare
Define the job before you ask for prices. A scope of works sets out, room by room, exactly what is being done, to what standard, with which fittings, and what is explicitly not included. Every builder then prices the same thing.
Give everyone the same pack, at the same time. Drawings, structural information, specification and scope, with a single deadline for returns.
Ask for the quote broken down. A single total tells you nothing. Section-by-section pricing shows where the differences actually sit.
Question the allowances rather than the total. "What have you allowed for the glazing?" is a better question than "can you do it cheaper?"
What good tender information contains
| Document | What it settles |
|---|---|
| Technical drawings | Construction detail, not just layout |
| Structural information | Beams, foundations and what the builder must install |
| Specification | Materials and standards |
| Schedule of works | Room-by-room list of every task |
| Named allowances | What is included for kitchens, glazing and finishes |
| Clear exclusions | What is genuinely not in the price |
Check my builder's quote
Check my builder's quote →