How to Budget for a Home Extension in 2026
The difference between a smooth extension project and a stressful one usually comes down to budgeting. Homeowners who budget correctly rarely run out of money mid-build. Those who guess often do. This guide walks you through building a realistic budget.
Cost breakdown
The budget formula
Build cost + kitchen/bathroom + professional fees + hidden costs + 15% contingency = realistic budget. Never skip the contingency — at least one unexpected cost will arise during the build.
Financing options
Savings: the cheapest option, no interest payments. Remortgage: typically the lowest interest rate for large sums. Home improvement loan: unsecured, quick to arrange, higher rates. 0% credit cards: useful for materials purchases under £10,000. Builder payment plans: some builders offer staged payments.
When to start
Allow 6–12 months from deciding to extend to the builder starting on site. This includes design (2–8 weeks), planning (0–12 weeks), structural and Building Regs (2–4 weeks), builder procurement (4–8 weeks), and builder lead time (4–12 weeks). Start the design process as early as possible.
| Category | % of Total Budget | Example (£80,000 total) |
|---|---|---|
| Build cost | 55–65% | £44,000–£52,000 |
| Kitchen/bathroom | 12–18% | £10,000–£14,000 |
| Professional fees | 4–7% | £3,000–£5,600 |
| Hidden costs | 5–8% | £4,000–£6,400 |
| Contingency | 12–15% | £9,600–£12,000 |
Prices based on Dorset rates, 2026
Frequently asked questions
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