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.
Planning the work? Check what it usually costs, or work out your own estimate. The free quote form is at the top of this page.
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 regional cost model · 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.