Extension costs in the UK vary enormously depending on location, specification, ground conditions, the complexity of the design and the quality of contractor you choose. Any figure you see online should be treated as a starting point for conversation rather than a reliable budget number. That said, you need a starting point, and this guide aims to give you one that's based on realistic current costs rather than optimistic marketing numbers.
All figures below are approximate 2025 costs, inclusive of VAT at 20%, for standard specification in London and the South East. Outside London and the South East, you can generally expect costs to be 15-25% lower, though this varies considerably by region and by trade availability in your area.
Single-Storey Extension
A single-storey rear or side extension is the most common type of domestic extension. It typically involves new foundations, blockwork or timber frame walls, a flat or pitched roof, windows and doors, floor finishes, and connection to the existing house.
| Size | Basic specification | Mid-range | High spec |
|---|---|---|---|
| 15m2 (approx 3x5m) | £35,000-£45,000 | £45,000-£60,000 | £65,000-£85,000+ |
| 25m2 (approx 5x5m) | £55,000-£70,000 | £70,000-£90,000 | £95,000-£130,000+ |
| 40m2 (larger kitchen-diner) | £80,000-£100,000 | £100,000-£130,000 | £140,000-£180,000+ |
These are build-only costs. Add professional fees (typically £5,000-£15,000 for architect, structural engineer and planning), kitchen or bathroom if the extension includes one (£5,000-£50,000 depending on specification), and a 15% contingency on top of the total.
Double-Storey Extension
A double-storey extension adds significant space but doesn't simply double the cost of a single-storey: the foundations, footings and overhead costs are shared. As a rough rule, a double-storey extension costs around 50-75% more than a single-storey of the same footprint, not double.
| Size (per floor) | Basic specification | Mid-range |
|---|---|---|
| 15m2 per floor (30m2 total) | £55,000-£70,000 | £70,000-£95,000 |
| 25m2 per floor (50m2 total) | £80,000-£105,000 | £105,000-£145,000 |
Kitchen-Diner Extension
Perhaps the most popular extension type. The construction cost depends on footprint and specification, but the kitchen itself adds significantly to total cost. A basic fitted kitchen runs £5,000-£15,000 installed. Mid-range, £15,000-£35,000. High-end, £35,000-£100,000 and beyond. Budget separately for this.
Rooflights are frequently included in kitchen extensions and add significant cost: a structural rooflight costs £800-£2,500 for the unit, plus installation.
Side Return Extension
Common in Victorian terraced houses, a side return fills in the narrow alley at the side of the kitchen. They're relatively compact (typically 8-15m2) but disproportionately impactful on kitchen space.
A typical side return costs £30,000-£60,000 including the new roof (usually glazed or with rooflights), bifold or sliding doors, and connection to the existing space. The complication is often party walls and shared boundaries, which can add surveyor fees.
What Drives Cost
Several factors push extension costs up significantly:
- Ground conditions: Poor ground (clay, made ground, proximity to trees) requires deeper or more complex foundations
- Structural complexity: Large spans, steel beams, multiple levels
- Roof type: Flat roofs are generally cheaper than pitched; glazed roofs are more expensive
- Materials specification: Matching existing materials on older properties can be significantly more expensive than modern alternatives
- London premium: Labour costs in London are typically 30-40% higher than the national average
- Party wall complications: Disputed party wall notices add time and cost
- Access: Restricted access (narrow side passages, no rear access) increases costs for materials delivery and scaffolding
Using Cost Per Square Metre
A rough rule of thumb for UK extension costs in 2025: basic specification runs £2,000-£2,500 per square metre, mid-range £2,500-£3,500 per square metre, and high specification £3,500-£5,000+ per square metre. In London, add 25-40% to these figures.
These are build costs only, excluding professional fees, kitchen/bathroom fit-out, and VAT. Always use them as a starting point for conversations with contractors and architects, not as a fixed budget number.
Most important step: Get three quotes against the same, fully detailed specification before committing to any budget figure. The spread between properly like-for-like quotes tells you a lot about both the market and the contractors.