How to Plan a Kitchen Extension Budget (2026 UK)
A kitchen extension is one of the best-value ways to transform a home, but it is also where budgets slip — usually because the kitchen, glazing and groundworks were never priced properly at the start. In 2026 the build alone costs £1,800–£2,600 per m², so a typical 20m² rear extension runs £40,000–£55,000 before the kitchen goes in. This guide shows how to set a realistic total, split the build from the fit-out, and hold the right contingency so the project lands on budget.
The Five Buckets Every Kitchen Extension Budget Needs
A budget that only counts "the building work" is the classic reason a kitchen extension overruns. Split your money across these five buckets before you speak to a builder, and price the big variables — glazing and the kitchen — at design stage rather than after the shell is up.
1. The shell (build cost)
Foundations, walls, roof, insulation and a basic internal finish at £1,800–£2,600 per m². This is the biggest single number and the one most quotes lead with.
2. Glazing
Bi-fold or sliding doors (£2,500–£5,500), a roof lantern (£1,500–£3,000) or rooflights. The biggest variable — easily £8,000 on a £40,000 job.
3. The kitchen itself
Units, worktops and appliances at £8,000–£25,000. Rarely included in the per-m² build rate, so budget it as its own line from day one.
4. Fees and Building Regs
Drawings, a structural engineer, Building Control and any party wall costs — usually 5–10% of the build. Cheap to overlook, awkward to find later.
5. Contingency
Hold 10–15% of the build back for what the ground reveals: drains, poor foundations or a manhole in the footprint. Do not spend it on upgrades.
Mind the ceiling price
Check the top sale price for your street. Spending £75,000 on a home that caps at £400,000 risks over-improving — the extension should add value, not just cost.
Kitchen extension build cost by size — 2026
Finished shell only, before the kitchen fit-out. Add £8,000–£25,000 for the kitchen.
Source: BestBuilders 2026 quote data · 519 UK towns
Full 2026 budget rate card
| Line item | Typical cost (2026) | Notes |
|---|---|---|
| Shell only (watertight, no fit-out) | £1,200–£1,600 per m² | You finish internally later |
| Standard specification, finished | £1,800–£2,600 per m² | Most common quote basis |
| High spec, glass-led | £2,600–£3,500 per m² | Large glazing, premium finish |
| Kitchen fit-out (units, worktops, appliances) | +£8,000–£25,000 | Budget as a separate line |
| Bi-fold or sliding doors (3m) | +£2,500–£5,500 | The most common upgrade |
| Roof lantern (1.5m x 1m) | +£1,500–£3,000 | Transforms light in a deep-plan room |
| Structural steel and knock-through | +£1,500–£4,000 | Opening up to the existing house |
| Groundworks and drainage moves | +£800–£2,500 | Sloping sites, moved gullies |
| Fees (design, engineer, Building Control) | 5–10% of build | Party wall extra if attached |
| Contingency | 10–15% of build | Ring-fence it — do not pre-spend |
A worked example: 20m² rear kitchen extension
| Bucket | Budget |
|---|---|
| Build shell, 20m² at £2,200/m² | £44,000 |
| Bi-fold doors + roof lantern | £6,500 |
| Kitchen fit-out (mid-range) | £14,000 |
| Fees and Building Regs | £3,500 |
| Contingency (12%) | £5,300 |
| Total budget | £73,300 |
Swap in your own figures, but keep the shape: the headline build number is only about 60% of what you will actually spend once glazing, the kitchen, fees and contingency are added.
Regional price differences
| Region | Against UK average |
|---|---|
| London | +20–30% |
| South East and Southern | +10–20% |
| Midlands and Eastern | Around the average |
| North West, Yorkshire, North East | −5–15% |
| Wales and Scotland | −5–15% |
Where kitchen extension budgets go wrong
- Kitchen forgotten — the build price rarely includes the units and appliances going into it. Budget £8,000–£25,000 separately.
- Glazing priced late — bi-folds and a lantern can add £8,000 after the shell is up. Decide the spec at design stage.
- Drainage discovered on site — a manhole in the footprint costs £800–£2,500 to divert. A pre-build drain survey removes the surprise.
- No contingency — without 10–15% held back, the first ground-condition problem stalls the job.
- Over-improving — spending past your street's ceiling price rarely returns at resale.
Choosing the builder: checklist
- Two recent kitchen extensions you can visit or phone about — not just general building work.
- £2m+ public liability insurance, verified, and a named Building Control route.
- A quote that itemises shell, glazing, steel and fees separately so you can compare like for like.
- A written stage-payment schedule — never more than 10–15% up front.
- Three itemised quotes on identical drawings so the numbers are genuinely comparable.
Red flags
- A quote that lumps everything into one number with no breakdown.
- The kitchen fit-out or steel quietly left off the scope.
- Pressure for a large cash deposit before any work starts.
- No mention of Building Regulations or a party wall agreement where one is needed.
Common Questions
Related Guides
More kitchen extension planning and cost guides.