Phase 1 · Planning & Permissions · Task 13 of 17
Budgeting for Your Extension: What It Really Costs
Learn how to budget properly for a single-storey extension, why builder quotes cover only part of the total, and how to protect yourself from overruns.
Prerequisites
Complete these first
Running out of money mid-build is the most common disaster in self-managed extensions. Not structural failure. Not planning refusal. Not a rogue builder. Just the slow, grinding realisation that the numbers don't add up, and you're only halfway through.
It happens because builders quote for building work. Not for the architect. Not for the planning fees. Not for steelwork, glazing, electrics, plumbing, screed, tiling, garden reinstatement, or the dozen other cost lines that appear between day one and completion. The builder's price is one line item in a much larger budget, and if you treat it as "the cost," you will run short.
What a Builder's Quote Actually Covers
A builder quoting for a single-storey rear extension is pricing labour and core materials for the physical structure: foundations, blockwork, DPC, roof structure and covering, insulation, plasterboarding, and plastering. Some builders include basic drainage. Some don't.
What the quote almost never includes:
- Architect fees
- Planning application and building control fees
- Structural engineer calculations
- Structural steelwork (RSJs and lintels)
- Windows, bifold doors, and glazing
- Electrics (first fix and second fix)
- Plumbing and heating
- Screeding
- Tiling and flooring
- Decoration
- Garden reinstatement and soakaway drainage
- Utility meter relocation
- Skip hire and waste removal
That list adds up to 55-65% of the total build cost. The builder's structural quote covers the remaining 35-45%, but those figures still exclude the kitchen itself. Kitchen units, appliances, and worktops sit entirely outside the builder's scope and are priced and sourced separately. Factor the kitchen in, and the builder's quote typically represents 25-40% of your true all-in spend. This is not a failing on the builder's part. It's the nature of the work: builders build. The rest falls to you.
The Numbers That Matter
£2,500 – £3,600 per m²
The all-in cost of a single-storey rear extension in the UK: professional fees, builder's work, specialist trades, finishes, garden reinstatement, AND kitchen units, appliances and worktops included. The wide range reflects regional differences (Midlands and North at the lower end; London and the South East at the upper end) and kitchen specification.
Industry consensus puts the total build cost of a straightforward single-storey extension (covering the builder's structural work, specialist trades, steelwork, windows, professional fees, and garden reinstatement) at £2,200 – £2,500. For quality specification and London/South East locations, that rises to £2,800 – £3,000.
But those figures don't include the kitchen. Add the kitchen (units, appliances, worktops, and fitting) and the true all-in cost rises to £3,000 – £3,600 and £2,500 – £3,000.
For a 25m² extension, that's roughly £62,500 – £90,000 all-in. For 40m², it's £100,000 – £144,000. These are real numbers, not scare stories. And if you've been mentally anchoring to a builder's quote of £35,000 – £50,000, the gap is enormous.
Regional variation
Where you live changes these numbers more than almost any other factor.
What the numbers look like in practice
A worked example for a 30m² single-storey rear extension in the Midlands at moderate specification. The middle column gives a modelled 2024 reference point; the right-hand column gives the typical 2026 UK low-to-high range for the same line, widened by current market research so you can stress-test a builder's quote against today's market rather than a snapshot:
| Cost line | Modelled 2024 baseline | Typical 2026 UK range | Share of build |
|---|---|---|---|
| Builder (structure, plasterboard, plastering) | £28,000 | £25,000-£38,000 | 40% |
| Structural steelwork | £5,500 | £5,500-£11,000 | 8% |
| Windows and external doors | £6,000 | £5,000-£12,000 | 9% |
| Electrics (first and second fix) | £5,000 | £4,500-£7,500 | 7% |
| Plumbing and heating | £6,000 | £6,000-£12,000 | 9% |
| Screed, tiling, and flooring | £5,500 | £4,500-£9,000 | 8% |
| Decoration | £2,000 | £1,500-£3,500 | 3% |
| Garden reinstatement | £4,000 | £3,000-£7,000 | 6% |
| Professional fees (architect, SE, planning, BC) | £6,000 | £5,000-£9,000 | 9% |
| Skip hire and waste | £2,000 | £1,500-£3,000 | 3% |
| Build cost total (exc. kitchen) | £70,000 | £62,000-£112,000 | 100% |
| Kitchen (units, appliances, worktops, fitting) | £12,000 | £10,000-£40,000+ | n/a |
| True all-in total | £82,000 | £72,000-£150,000+ | n/a |
The builder's quote is 40% of the build cost but only 34% of the true all-in. That gap is why the builder's quote always feels smaller than the final bill. The 2026 ranges are wide because specification drives cost far more than square metres. Bifold door sets, premium kitchens, and complex steel arrangements all push the upper end. The Building Cost Information Service tender price index rose roughly 24% from 2021 to 2026 (per Costmodelling data), with structural steel running harder at 40-60% above its 2021 figure before partially retreating. Re-check the ranges annually for anything you plan to commit to.
Do this first
London carries a 10-25% premium over the UK average. The North and Midlands are genuinely cheaper, partly because tradesperson day rates are lower (£180 – £250 vs £250 – £350) and partly because land values and overheads are lower. If a London builder quotes you £2,500 per m² for the build alone, that's competitive. The same quote in Leeds would be expensive.
What this guide covers
- 01Where the Money Goes
- 02Building a Realistic Budget
- 03Contingency: Why Is the Minimum
- 04Common Budget Traps
- 05Payment Structure and Cash Flow
- 06Sanity-Checking Your Budget
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