Rear Extension Cost Calculator UK
Get a 2026 rear extension cost estimate in 30 seconds. Set the depth and width, pick your spec and glazing, add a structural knock-through — real UK build rates including VAT. No email required.
Estimate Your Rear Extension Cost in 30 Seconds
6 steps. Covers foundations, shell, roof, glazing, plastering, decoration and VAT — plus the steel beam if you open the back of the house up.
How This Calculator Works
Our rear extension calculator multiplies your depth and width into a floor area, applies real 2026 spec-level build rates, then adds the knock-through, glazing and your region.
Build Rates (per m², complete)
- Basic spec — £1,700–£2,000/m²
- Standard spec — £1,900–£2,600/m²
- Premium / glass-heavy — £2,800–£3,500/m²
- Structural knock-through with steel beam +£3,500 typical
- Bifold/sliding doors +£5,000 · structural glazing +£12,000
Rates include foundations, shell, roof, glazing, plastering, decoration, labour and VAT.
Assumptions & Adjustments
- Permitted development: 3m depth (semi/terraced) or 4m (detached) without planning; 6m/8m via prior approval
- Add 10–15% professional fees and 10–15% contingency
- Party wall agreements £700–£1,500 per affected neighbour — almost always relevant on terraces
- Region multipliers: London +27% … Northern Ireland −12% (Midlands baseline)
- Estimate only — a firm quote always follows a site survey
The deeper you go past the PD limits, the more the planning route shapes your timeline.
Typical Rear Extension Costs (2026)
Complete build incl. foundations, shell, roof, glazing, decoration & VAT, Midlands baseline. Professional fees (10–15%) and contingency extra.
| Scenario | Size | Cost range |
|---|---|---|
| Standard spec | 3m × 5m (15 m²) | £28,500–£39,000 |
| Standard spec | 4m × 5m (20 m²) | £38,000–£52,000 |
| Standard + knock-through & steel | 4m × 6m (24 m²) | £49,100–£65,900 |
| Standard spec (prior approval depth) | 6m × 5m (30 m²) | £57,000–£78,000 |
| Premium / glass-heavy | 4m × 6m (24 m²) | £67,200–£84,000 |
Want the full method and per-material detail? Read the complete 2026 roofing cost guide.
Frequently Asked Questions
How much does a rear extension cost in 2026?
How far can I extend without planning permission?
What does the knock-through and steel beam cost?
Do I need a party wall agreement?
How long does a rear extension take?
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