1. A proper extension quote is several pages, not one line
A single-storey rear extension in the UK is a substantial project. A quote that fits on one page with one total is almost never complete. Expect a scope breakdown by stage: enabling works, substructure, superstructure, roof, openings, first fix, second fix, finishes and externals.
2. Substructure: foundations and drainage
Foundation type and depth depend on soil, trees and existing drainage. The quote should reference building control approval, any engineer's design, and how drainage diversions or build-overs are handled. "Foundations included" with no specification is too vague.
3. Superstructure: walls, steels and roof
Look for the wall build-up (block, insulation, brick or render), steel sizes per the structural engineer, lintels, roof structure, insulation, roof covering and rooflights. If steels are "provisional", the final cost can move significantly.
4. Openings, glazing and bifolds
Doors and windows are often quoted as a PC ("prime cost") sum or supply allowance. Push for named products or a clearly stated budget, and confirm whether fitting, sealing and trims are included.
5. First and second fix
First fix covers plumbing, electrics, underfloor heating pipework and carpentry behind the plaster. Second fix covers sockets, switches, sanitaryware, kitchen fit and final carpentry. Both stages should be itemised, not lumped into "M&E allowance".
6. Finishes: plaster, flooring, decoration
Plastering, screed, tiling, flooring and decoration are often underestimated. Confirm what's included as standard and what is a provisional sum. Kitchens, wardrobes and bathrooms are usually separate.
7. Externals and making good
Removing spoil, reinstating the garden, patio works, soffits, gutters, downpipes and external decoration are real costs that get forgotten. Confirm what state the garden and driveway are left in.
8. Professional and statutory costs
Architect, structural engineer, planning fees, building control, party wall surveyor and warranty are usually outside the builder's quote. The Renovation Budget Planner includes a cost-categories sheet so these don't get missed.
9. VAT, payment schedule and contingency
Confirm VAT status. Agree a sensible payment schedule tied to completed stages with a small retention. Hold a 10–15% contingency for variations.
Use this on your actual extension quote
Run any extension quote through the free Builder Quote Checklist. For a deeper line-by-line score and a comparison grid across multiple builders, use the Homeowner Quote Checker Pack.
Use this on your actual project
The free Builder Quote Checklist gives you a short printable pack to sanity-check any UK builder quote. The Homeowner Quote Checker Pack goes further with the exact ground covered in this article.
The Builder's Brief UK provides practical homeowner guidance based on real trade experience. It is not legal, financial, architectural, structural engineering, planning or building-control advice, and is not a replacement for project-specific guidance from suitably qualified professionals.