Quote check
Why are my builder quotes so different?
Quotes for the same extension often differ by tens of thousands of pounds because the builders are not really pricing the same job. They read the drawings differently, leave out different items, treat VAT differently and put a different price on risk. Once you line them up like for like, honest quotes usually land much closer together.
The seven usual reasons
- Different scope. One quote includes the kitchen, electrics, plumbing, flooring and decorating. Another stops at plastered walls and leaves the rest to you.
- Provisional sums. A provisional sum is a placeholder for work the builder cannot price yet, such as foundations or drainage. A quote full of low provisional sums looks cheap and grows once work starts.
- VAT. Some quotes add 20% VAT on top, some include it, and builders below the VAT registration threshold do not charge it at all.
- Construction type. A lightweight glazed room or conservatory costs far less than an insulated extension built to Building Regulations, but it is not the same thing. It may not be comfortable all year, and buyers and lenders can treat it differently.
- Groundwork assumptions. Foundations, drains and tree roots stay hidden until digging starts. One builder allows for deeper foundations; another assumes the minimum and charges extra later.
- Preliminaries. Scaffolding, skips, site welfare, supervision and insurance. Some builders list them, some spread them through their rates, and some leave them out.
- Workload and risk. Busy builders price high because they do not need the job. Others price low to win it and recover the difference through extras.
Finish level alone moves the price a long way
The same 25 m² single-storey rear extension costs very different amounts depending on the specification. If one builder has priced basic doors, windows and finishes and another has priced large sliding doors and premium flooring, their quotes will be far apart and both can be fair.
| Finish | Build cost | Per m² |
|---|---|---|
| Budget | £51,000 to £63,000 | £2,000 to £2,500 |
| Mid-range | £63,000 to £78,000 | £2,500 to £3,100 |
| Premium | £81,000 to £99,000 | £3,200 to £4,000 |
How to make quotes comparable
- Send every builder the same drawings and the same written specification.
- Ask for an itemised price rather than a single figure.
- List every exclusion and provisional sum side by side.
- Add a sensible allowance for anything a quote leaves out.
- Convert every quote to a price including VAT.
- Compare the results with the planning range for your size, finish and region in the extension cost calculator.
Our free quote comparison worksheet takes you through this line by line. Builders and surveyors can also price an extension element by element with the builder's estimate tool and see where a tender sits against the planning range.
When a low quote is a warning
- It is far below the others and you cannot see why
- Much of the price sits in provisional sums
- There is no mention of building control or structural calculations
- The builder wants cash, or a large payment before work starts
- There is no written specification, contract or guarantee
A low quote is not automatically a bad one, but it should be explained. Ask the builder to confirm in writing what is included.