Average Cost of a Two-Storey Extension in 2026 UK
A two-storey extension is one of the best-value ways to add space in 2026, because you pay for one set of foundations and one roof tie-in but gain two floors. Expect £1,900–£2,900 per m² of finished floor area, or roughly £60,000–£130,000 for a typical build once fees and finishes are included.
Two-storey extension costs — 2026 UK at a glance
- Build cost: £1,900–£2,900 per m² of finished floor area (both floors counted).
- Typical project: £60,000–£130,000 for a 20–50 m² extension.
- Professional fees: add 10–15% for architect, structural engineer and building control.
- London & South East: add 20–35% to every figure on this page.
- Better value than single-storey: the second floor typically costs 25–35% less per m² than the first.
Bottom line: budget the build cost, then add 10–15% fees and a 10% contingency. A 40 m² two-storey rear extension realistically lands around £88,000–£136,000 fully finished outside London.
Two-storey extension cost by floor area (2026 UK)
Figures exclude VAT relief cases and assume standard ground conditions. Add 20–35% in London and the South East, and more if you need underpinning or a basement tanking solution.
What actually moves the price
- Foundations and ground conditions — clay, trees or a high water table can add £5,000–£15,000 before you see a single brick.
- Steelwork — opening up the ground floor to the existing house needs beams; expect £3,000–£9,000 supplied and fitted.
- Roof tie-in — matching an existing pitched roof is far dearer than a simple flat-roof junction.
- Glazing — bi-folds or sliding doors run £2,500–£9,000 per opening in 2026.
- Kitchen or bathroom fit-out — often the single biggest variable, from £6,000 to £40,000+.
- Planning and party wall — a full planning application plus party wall awards typically adds £1,500–£4,000.
Tip: ask every builder to quote to the same written specification. Two quotes that look £20,000 apart are usually pricing different levels of finish, not different levels of skill.
Frequently asked questions
Expect GBP 1,900-2,900 per square metre of finished floor area, counting both floors. A typical 40 square metre two-storey rear extension costs GBP 88,000-136,000 fully finished outside London, including professional fees.
Yes. You pay once for foundations, scaffolding and the roof tie-in but gain two floors, so the upper floor typically costs 25-35% less per square metre than the ground floor. That is why two-storey extensions usually offer better value per square metre.
Usually yes. Two-storey rear extensions can fall under permitted development in limited circumstances, but the rules on depth, height, boundary distance and materials are tight, and they do not apply to flats, listed buildings or many conservation areas. Most homeowners submit a full householder application.
Allow 14-22 weeks on site for a typical build, plus 8-12 weeks for planning determination and 4-8 weeks for design and structural drawings beforehand. A realistic end-to-end timeline is 7-11 months.
Yes, standard-rate VAT applies to most extension work on an existing home. Reduced or zero rates can apply to new builds, properties empty for two or more years, and certain accessibility adaptations. Check your eligibility with HMRC before assuming a discount.
Kitchen and bathroom fit-out, glazing and ground conditions. The structural shell is fairly predictable; the fit-out specification is where two otherwise identical projects can differ by GBP 30,000 or more.
Sources used in this guide
Methodology: Cost ranges reflect quotes collected through the BestBuilders network and published trade pricing for Q3 2026 UK, excluding VAT relief cases. Regional variation of 20-35% applies in London and the South East. Last fact-checked: .