Budgeting for a single storey extension in the UK is less about finding one magic number and more about making sure you have a line for every cost the project will actually generate. Most homeowners who go over budget didn't get unlucky — they simply never had design fees, VAT, making good or contingency in the spreadsheet to begin with.
This guide walks through how to build a realistic budget for a UK single storey rear or side extension, in plain English. Use it alongside the free Builder Quote Checklist and the Renovation Budget Planner so every line below ends up in your own numbers, not just the builder's.
1. Start with a sensible £/m² range, not a single figure
As a working benchmark in 2026, a standard single storey extension in the UK lands around £2,200–£3,200 per m² outside London, and £2,800–£3,800+ per m² in London and the South East. Use the range as a sanity check, not a quote — your access, ground conditions, glazing and kitchen spec move the number significantly.
2. Build the budget in five buckets
- Design and approvals — architect, structural engineer, planning, building control, party wall.
- Shell — foundations, walls, steels, roof, glazing, external finishes.
- Making good — knock-through, re-plastering, flooring across old and new, moving services.
- Fit-out — kitchen, appliances, electrics, plumbing, tiling, decoration.
- Project costs — skip, scaffolding, surveys, temporary kitchen, contingency, VAT.
3. Don't forget the pre-build fees
Before a single brick is laid, expect £4,000–£10,000 in design and approval fees on a typical extension. These rarely sit inside the builder's quote and are easy to miss.
4. Treat VAT as a separate decision
Most domestic UK extensions are standard rated at 20% VAT. On a £55,000 build that's £11,000 — budget-breaking on its own. Ask every builder whether their figure is inclusive or exclusive of VAT before you compare quotes.
5. Add a real contingency (10–15%)
Ground works find surprises. Old houses hide rotten joists, buried drains and out-of-square walls. A 10–15% contingency isn't pessimism — it's normal UK renovation maths.
6. Track the "easy to forget" line items
- Skip hire and skip permits
- Scaffolding
- Temporary kitchen / eating out
- Storage of furniture
- Reinstating garden, lawn or drive
- Curtains, blinds and furniture for the new room
How to apply this to your own project
Use the Renovation Budget Planner to put your own numbers against every bucket above, then run each builder quote through the free Builder Quote Checklist so you can see exactly which lines are missing. For a full cost breakdown by line, see the single storey extension costs guide.
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 Renovation Budget Planner 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.