London Extension Costs 2025: Real Project Data
Actual costs from 5,000+ London extension projects. Ground floor, loft, side return, and two-storey extensions with free instant calculator.
Average London Extension Costs 2025
- Per m²: £3,100 - £4,300/m² (standard spec)
- Ground floor rear: £55,000 - £120,000 (15-30m²)
- Loft conversion: £45,000 - £85,000 (15-25m²)
- Side return: £55,000 - £85,000 (12-20m²)
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London Extension Cost Per Square Meter
Based on 5,000+ projects completed through Multiply.build in 2023-2024, London extension costs range from £3,100 to £4,300 per m² for standard specification.
| Specification | Cost per m² | 20m² Extension |
|---|---|---|
| Basic | £2,800 - £3,100 | £56k - £62k |
| Standard | £3,100 - £3,700 | £62k - £74k |
| Premium | £3,700 - £4,300 | £74k - £86k |
| Luxury | £4,300 - £5,500+ | £86k - £110k+ |
* Costs include construction only. Design fees, planning, and party wall separate (see below).
Ground Floor Rear Extension Costs
The most common extension type in London. Typical sizes: 15-30m².
3m x 5m Extension (15m²)
- Standard spec: £55,000 - £65,000
- Premium spec: £65,000 - £75,000
- Includes: Single glazed doors, standard tiles, basic kitchen
4m x 6m Extension (24m²)
- Standard spec: £75,000 - £90,000
- Premium spec: £90,000 - £105,000
- Most popular size for family kitchen extensions
6m x 5m Extension (30m²)
- Standard spec: £95,000 - £115,000
- Premium spec: £115,000 - £135,000
- Often includes: Bifold doors, underfloor heating, premium finishes
Note: Extensions over 3m deep (terraced/semi) or 4m (detached) typically require planning permission. See our PD rights guide.
Loft Conversion Costs London
Loft conversions add bedroom space without extending the footprint. Typical sizes: 15-25m².
| Conversion Type | Typical Cost | Size |
|---|---|---|
| Velux/Rooflight only | £40k - £55k | 15-20m² |
| Rear dormer | £50k - £70k | 18-25m² |
| Hip-to-gable + rear dormer | £60k - £85k | 25-30m² |
| Mansard (full rebuild) | £80k - £130k | 30-40m² |
* All include new stairs, structural work, ensuite bathroom, and bedroom fit-out.
Additional Loft Costs
- Party wall agreements: £1,000 - £2,500 per neighbour
- Structural engineer: £800 - £1,500
- Building regulations: £1,000 - £1,500
Side Return Extension Costs
Popular in Victorian terraced houses. Typically 1.2m wide alleyway converted to usable space.
Single-Storey Side Return (Kitchen)
- Size: 12-18m² (1.2m wide x 5-6m deep)
- Cost: £55,000 - £75,000
- Includes: Full-height glazing, kitchen, bi-fold/sliding doors
Two-Storey Side Return
- Size: 24-36m² (12-18m² per floor)
- Cost: £95,000 - £135,000
- Adds: Ground floor kitchen + first floor bedroom/bathroom
Side return planning: Most side returns in London require planning permission due to 7m boundary rule. Budget extra 3-4 months for planning approval.
Two-Storey Rear Extension Costs
Maximum value addition but higher cost. Typical sizes: 30-60m² total (15-30m² per floor).
3m Deep Two-Storey
- Size: 30m² total (15m² per floor)
- Cost: £95,000 - £125,000
- PD-compliant: Most cases (if not in conservation area)
6m Deep Two-Storey
- Size: 60m² total (30m² per floor)
- Cost: £180,000 - £250,000
- Requires: Planning permission (always)
What's Included in These Costs?
Typical Build Cost Includes:
- Foundations and structural work
- External walls, roof, windows, doors
- Internal walls, plastering, decoration
- Electrics and plumbing (first fix)
- Flooring (tiles/wood)
- Radiators/underfloor heating
- Basic kitchen units (if kitchen extension)
NOT Included (Budget Separately):
- Design fees: £1,295 - £4,500 (see below)
- Planning fees: £258 - £462 (if required)
- Building regulations: £1,000 - £1,500
- Party wall agreements: £1,000 - £2,500 per neighbour
- Structural engineer: £800 - £1,800
- Kitchen appliances: £2,000 - £10,000+
- Premium tiles/fixtures: Variable
Professional Fees Breakdown
| Service | Our Price | Market Range |
|---|---|---|
| Single storey extension | £1,295 | £1,500 - £3,500 |
| Two storey extension | £1,795 | £2,500 - £5,000 |
| Loft conversion | £1,495 | £2,000 - £4,000 |
| Side return + rear | £1,795 | £2,500 - £5,500 |
* Includes: Architectural drawings, planning application (if needed), structural calculations, building regs.
Cost Variations by London Borough
Build costs vary 5-15% across London based on:
- Builder availability (higher demand = higher prices)
- Access (narrow streets, parking restrictions add cost)
- Planning requirements (conservation areas = more oversight = higher cost)
Higher Cost Boroughs
Westminster, Kensington & Chelsea, Camden, Islington typically 10-15% above average due to stricter planning requirements and conservation area restrictions.
Lower Cost Boroughs
Outer London (Croydon, Barking, Havering, Bexley) typically 5-10% below average due to easier access and fewer planning restrictions.
How to Reduce Extension Costs
- 1
Stay Under PD Limits
Building under permitted development saves £258-462 in planning fees and 2-3 months time. More importantly, it avoids risk of planning rejection which can add thousands in revised designs.
- 2
Simple Roof Design
Flat roofs with simple parapet cost £40-60/m². Pitched roofs with complex valleys cost £90-140/m². On a 25m² extension, that's £1,250 vs £3,500.
- 3
Standard Glazing
Bi-fold doors: £4,000-7,000 for 3m opening. Sliding doors: £2,500-4,500. Fixed glazing with standard door: £1,500-2,500.
- 4
Get 3+ Builder Quotes
Builder quotes for the same project often vary 15-25%. We can introduce you to vetted builders who know your design.
- 5
Time Your Project
Starting construction in October-February (off-peak) can save 5-10% vs peak season (April-August).
Get Your Exact Extension Cost
Our free calculator uses real London project data to give you an accurate estimate in 2 minutes. No email required.
Frequently Asked Questions
How accurate are these costs?
Based on real completed projects 2023-2024. Your actual cost may vary ±10% based on specification choices, site conditions, and builder selected. Use our calculator for a more specific estimate.
Do costs include VAT?
No. Add 20% VAT unless your extension creates a separate dwelling (0% VAT) or you're renovating an empty property (5% VAT). Residential extension work is standard-rated 20%.
What's the cheapest type of extension?
Per m², loft conversions are typically cheapest (£2,800-3,500/m²) because they use existing structure. Single-storey rear extensions cost £3,100-3,700/m². Two-storey extensions are most expensive per m² (£3,300-3,900/m²) due to additional structural requirements.
Can I save money doing work myself?
Not recommended for structural work. You can save on:
- Decoration (£1,500-3,000 saving)
- Kitchen installation if experienced (£500-1,500 saving)
- Removing old kitchen/garden (£800-1,500 saving)
Attempting structural work yourself often costs more when correcting mistakes.
How much does an extension add to property value?
Typical ROI: 70-100% of build cost in London. A £75k kitchen extension typically adds £50k-75k value. Best ROI areas:
- Kitchen extensions (80-100% ROI)
- Extra bedroom/bathroom (75-90% ROI)
- Loft with ensuite (70-85% ROI)
Summary
London extension costs in 2025:
- £3,100-4,300/m² for standard spec
- £55k-120k typical ground floor rear extension
- £45k-85k typical loft conversion
- £1,295-1,795 design fees (fixed price)
Use our free calculator to get an accurate estimate for your specific project with real London data.