An extension is a significant investment, often costing between £40,000 and £150,000 depending on your location and specification. In 2026, the UK building industry is facing a dual challenge: high demand for quality labour and a rapidly changing regulatory environment.
To ensure your project is a success, you need a builder who is organised, transparent, and technically proficient.
1. Start with the “Professional Trinity”
Before you look at aesthetic portfolios, verify the three pillars of a professional UK builder:
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Accreditation: Look for membership in the Federation of Master Builders (FMB) or the National Federation of Builders (NFB). These bodies vet their members for financial health and site safety.
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TrustMark: This is the Government-endorsed quality scheme. A builder with the TrustMark logo has been independently assessed for customer service and technical competence.
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Insurance: A professional must have Public Liability Insurance (typically £2m+) and Employer’s Liability Insurance. In 2026, it is also wise to ask if they have Contract Works Insurance, which covers the extension itself while it is under construction.
2. Technical Savvy: The 2026 Factor
Building regulations changed significantly in late 2025 and 2026. Your builder must be conversant with:
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Part L (Fuel and Power): New extensions must meet strict “U-values” for insulation. If your builder doesn’t mention thermal bridging or airtightness tapes, they may not be up to date with 2026 standards.
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The Planning & Infrastructure Bill: New 2026 reforms aim to streamline permissions. A good builder will know if your project qualifies for Permitted Development or if the new “Prior Approval” pathways are more suitable.
3. The “Three-Quote” Rule (But Look Beyond the Total)
Always obtain at least three written quotes. However, in 2026, the cheapest quote is often the riskiest.
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Quote vs. Estimate: An estimate is a guess; a quote is a fixed price. Demand a detailed quote that breaks down labour, materials, and VAT.
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Provisional Sums: Check for “provisional sums” (allowances for items not yet chosen, like tiles). If these are suspiciously low, your final bill will inevitably skyrocket.
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Payment Terms: A professional will never ask for 100% upfront. Expect a small deposit followed by staged payments linked to physical milestones (e.g., foundations poured, “watertight” stage, etc.).
Builder Vetting Checklist
| Question to Ask | Why It Matters |
| “Can I see a similar extension you finished in the last 12 months?” | Proof of recent, relevant experience. |
| “Who is your specialist for the ‘First Fix’ electrics and plumbing?” | Ensures they use qualified, certified sub-contractors. |
| “Do you provide a JCT or FMB contract?” | Protects you legally if the project overruns or disputes arise. |
| “How do you manage site waste and skips?” | In 2026, waste disposal is strictly regulated; you don’t want a mess left on your drive. |
4. The Culture Fit: Communication is King
You will likely be dealing with your builder for 3 to 6 months. If you find them difficult to reach during the quoting stage, it will only get worse once they are on-site.
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Single Point of Contact: Will you be speaking to the business owner or a dedicated site manager?
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Site Visit: Ask to see a “work in progress” site. Is it tidy? Are the workers wearing PPE? A disorganised site usually leads to a disorganised build.
5. Formalise Everything with a Contract
In 2026, a “handshake deal” is a recipe for disaster. Use a standard form contract such as the RIBA Domestic Building Contract or an FMB Contract.
This document should clearly state:
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Start and End Dates: Including what happens if the project overruns.
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Payment Schedule: Amounts and triggers.
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Defects Liability Period: Usually 6–12 months after completion where the builder returns to fix “snags” for free.
Final Thoughts
Knowing how to choose the right builder for your home extension is about balancing your gut instinct with cold, hard evidence. A builder who is proud of their work, transparent about their pricing, and knowledgeable about 2026 regulations is worth their weight in gold.
