The short answer
The key steps are: get at least three quotes from builders who have visited your site, check references from recent comparable projects, verify insurance and any trade membership, compare quotes on a like-for-like basis, and agree a written contract before any money changes hands. See extension costs and whether you need an architect to manage the procurement process.
Choosing the wrong builder is the single most common cause of extension projects going wrong — over budget, over time, or with poor quality. The right builder is not always the cheapest, and not always the one who is available immediately. This guide sets out the process for finding and selecting a builder who is right for your specific project, based on published guidance from trading standards, LABC and RICS.
Choosing a builder at a glance
- Minimum quotes Three from builders who have visited the site
- References At least two from comparable recent projects
- Insurance to check Public liability & employer’s liability (minimum £2m PLI)
- Trade membership FMB, NHBC, TrustMark or similar not compulsory but useful
- Contract Written contract before any money; avoid cash-only builders
- Deposit No more than 10–25% of contract value, tied to a programme
Before you start looking for builders
You need a design before you can get meaningful quotes. A builder who prices from a verbal description or a rough sketch is guessing, not quoting. At a minimum, you need planning drawings (if a planning application is needed) or a measured survey with a clear brief, plus any structural engineer’s drawings and calculations. Ideally, you have a full set of building regulations drawings so that all builders are pricing exactly the same scope. If you appoint an architect or designer, this is a normal part of their service. See do I need an architect for an extension? for when professional design input makes sense.
Finding candidates
Start with:
- Personal recommendations: neighbours, friends or family who have had similar work done recently. A first-hand account from someone in your area, on a comparable project, is the most reliable evidence.
- Trade body directories: the Federation of Master Builders (FMB), TrustMark and the NHBC (for new-build registered builders) maintain searchable directories of member firms. Membership does not guarantee quality but does mean the firm has agreed to a code of conduct.
- Local authority-recommended lists: some councils maintain lists of builders who have worked successfully on projects in the area.
| Check | What to ask / verify |
|---|---|
| References | Two recent comparable projects; visit if possible; ask about programme and budget |
| Public liability insurance | At least £2m cover; ask for the certificate |
| Employer’s liability insurance | Required by law if they employ workers on your site |
| VAT registration | Request the VAT number; most extension builders should be registered |
| Previous extensions | Ask to see completed projects of the same type as yours |
| Subcontractors | Who manages the trades (electrics, plumbing)? Are they directly employed or subbies? |
Comparing quotes
Only quotes based on the same drawings and specification are directly comparable. Go through each quote line by line:
- Does the quote include VAT? Is the builder VAT-registered?
- What is explicitly excluded (kitchen supply, flooring, decoration)?
- Is the price fixed or an estimate? What triggers a variation?
- What is the payment schedule? Avoid paying large sums upfront.
- What programme (timeline) is assumed? Is there a liquidated damages clause for late completion?
The contract
Always agree a written contract before any money changes hands. A suitable form for domestic work is the JCT Minor Works Building Contract (or the Homeowner Contract for smaller projects), which sets out the scope, price, programme, payment terms, insurance requirements and dispute resolution. An architect acting as contract administrator can certify payments and manage variations on your behalf. Do not rely on a handshake or a builder’s own terms on the back of a quote sheet. A written contract protects both parties and is the foundation for any dispute resolution. For further guidance on consumer rights in building contracts, Citizens Advice and trading standards publish accessible resources.
This page is general information about choosing a builder in England. Costs and contractor practices vary by region. Always take up references in person, check insurance certificates independently, and sign a written contract before making any payment.
Ready to compare extension quotes?
We can connect you with experienced extension builders who will visit your site and provide written, detailed quotes. No obligation, free to use.
Frequently asked questions
How many quotes should I get for a house extension?
At least three, from builders who have visited the site and are pricing from the same drawings. Fewer quotes make it harder to sense-check the price and identify outliers. Do not accept quotes based on a verbal description.
Should I choose the cheapest builder?
Not automatically. A significantly lower quote usually means something has been omitted or the specification is lower. Compare quotes line by line on a like-for-like basis and understand why the prices differ before deciding.
What insurance should an extension builder have?
At minimum, public liability insurance of £2m and employer’s liability insurance (legally required if they have employees). Ask for the certificates and check they are current before work starts.
What should a building contract for an extension include?
The scope of work, the contract sum, the payment schedule, the programme (timeline), the specification, what is excluded, how variations are agreed and priced, insurance requirements, and a dispute resolution mechanism. A JCT Minor Works or Homeowner Contract is suitable for most domestic extension projects.
Sources & further reading
- FMB (Federation of Master Builders) — Finding a reliable builder and checking membership
- TrustMark — Government-endorsed quality scheme for tradespeople
- LABC — Choosing contractors and the building regulations process
- RICS — Guidance on domestic building contracts and professional standards
This is general information about house extensions in England and is not planning, structural, legal or financial advice. Costs, timescales and outcomes vary with your design, ground conditions, specification and local authority. Always obtain written quotes and verify planning and building regulations requirements with your local planning authority before committing to any works.