Quick answer (updated October 2026): Not always. For most single-storey extensions, a detailed estimate from your planning or building regulation drawings (from £95 + VAT) is enough to set a budget and check quotes. A Bill of Quantities (from £200 + VAT) is worth it when you want several builders to price exactly the same thing, or the extension is large or complex.
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What Is a Bill of Quantities?
A bill of quantities (BoQ) is a document that lists every item of work in a project, measured and described in a standard way. Each item has a description, a quantity and a unit, organised by trade or element. Our BoQs follow NRM2. Builders price each line, which means every quote is built on exactly the same scope.
For a full explanation, see our guide: What Is a Bill of Quantities and Why Do You Need One?
When a BoQ Is Worth It for an Extension
A BoQ earns its cost when comparing like for like really matters. That is usually when:
- you are inviting three or more builders to tender and want quotes you can compare line by line;
- the extension is large, two storeys, or combined with a loft conversion or major internal changes;
- the design includes structural work, complex junctions or a high specification where gaps in pricing are likely;
- your lender, architect or contract administrator has asked for one;
- you want an agreed set of rates to value changes during the build.
When an Estimate Is Enough
For many homeowners, the question is really “what should this cost, and is my builder’s quote fair?” A detailed estimate answers that. It gives you an elemental cost breakdown you can set against each quote, and it costs less than a BoQ.
- Budget estimate, from £75 + VAT — early feasibility from outline drawings, typically within 15–20% of the final cost.
- Planning drawing estimate, from £95 + VAT — measured from planning drawings, typically within 10–15%.
- Building regulation estimate, from £150 + VAT — our most detailed estimate, typically within 5–10%.
If the extension is part of a wider refurbishment, a schedule of works can be a good middle ground: it lists the tasks by trade for builders to price, without the full measurement of a BoQ.
How the Options Compare
| Option | Best for | Starting price |
|---|---|---|
| Planning drawing estimate | Setting a budget and checking a quote | From £95 + VAT |
| Building regulation estimate | An accurate budget before committing to a builder | From £150 + VAT |
| Schedule of works | Refurbishments and mixed works for tender | Fixed-price quotation |
| Bill of Quantities | Competitive tenders on like-for-like terms | From £200 + VAT |
For typical extension build costs, see our house extension cost guide.
What Goes Into a BoQ for an Extension
A Bill of Quantities for a house extension is organised by trade or element, following NRM2. It typically includes:
- Preliminaries — site setup, welfare, skips, scaffolding and site management for the whole job.
- Measured works — foundations and groundworks, drainage, walls, floors, roof, windows and doors, internal finishes, and electrical and plumbing work, each measured in the right unit.
- Provisional sums — allowances for work that can’t be fully defined yet, such as unknown ground conditions or utility connections.
- Prime cost sums — allowances for items you will choose later, such as a kitchen or sanitaryware, with the builder pricing the fitting.
Using a BoQ During the Build
A BoQ keeps working after the tender. Because every item already has an agreed rate, the cost of any change can be calculated quickly and fairly, which avoids disputes. The same rates form the basis of interim payments, so you pay for work that is actually done. And if there is a disagreement about what was included, the BoQ is the agreed reference point. On an extension where the design is likely to change on site, that alone can justify the cost.
How to Decide
Start with your goal. If you need a figure to plan finance or test a design, get an estimate. If you have building regulation drawings and are about to go out to tender with several builders, a BoQ will make the quotes far easier to compare. If you’re unsure, send us your drawings and tell us what you’re trying to achieve. We’ll recommend the right document and give you a fixed-price quotation before we start.