Rear Extension London 2026: Complete Cost and Planning Guide
Everything you need to know about rear extensions in London for 2026. From costs and planning requirements to design options and permitted development limits.
Rear Extension 2026: Quick Summary
- Single-storey cost: £80k-100k (£3,200-3,700/m²)
- Two-storey cost: £140k-165k (£3,500-4,200/m²)
- Timeline: 4-6 months (single) or 6-9 months (two-storey)
- PD limits: 3-6m deep depending on house type (often restricted in London)
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Single vs Two-Storey Rear Extensions
The choice between single and two-storey rear extensions is one of the most important decisions you will make. It affects cost, planning, and how you use your home.
Single-Storey
- £80k-100k average cost
- May qualify for PD
- 4-6 month build time
- Ideal for kitchen-diners
Two-Storey
- £140k-165k average cost
- Usually needs planning
- 6-9 month build time
- Adds bedroom + living space
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Get Free EstimateRear Extension Costs in London (2026)
Average Total Costs
London rear extension costs have risen 3-4% from 2025 due to labour inflation:
| Extension Type | Size | Average Cost (2026) |
|---|---|---|
| Small single-storey | 15-20m² | £70k-80k |
| Medium single-storey | 20-30m² | £80k-100k |
| Large single-storey | 30-40m² | £100k-120k |
| Small two-storey | 30-40m² | £120k-145k |
| Medium two-storey | 40-50m² | £140k-165k |
| Large two-storey | 50-60m² | £165k-195k |
Cost Per Square Metre (2026)
London rear extensions typically cost £3,200-3,700 per m² for basic spec, rising to £3,700-4,500 per m² for high spec.
What is Included in Cost Per m²
- Foundations and groundworks
- Structure and roof
- Windows and doors
- Electrical and plumbing
- Heating and insulation
- Plastering and decoration
- Basic flooring
- Building control fees
Not included: Kitchen units, appliances, bespoke joinery, landscaping, or professional fees (architect, structural engineer, party wall).
Additional Costs to Budget For
- Architect fees: £5k-10k (8-12% of build cost)
- Structural engineer: £1.5k-3.5k
- Party wall agreements: £1k-2.5k (if applicable)
- Planning fees: £258-500 (if required)
- Kitchen: £10k-30k+ depending on spec
- Contingency: 10-15% of total budget
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Free instant report showing conservation areas, Article 4 directions, and flood zones for any UK address.
Get Free Property ReportPermitted Development Size Limits
You can build rear extensions under permitted development (PD) without planning permission, but size limits apply and many London properties have restrictions.
Single-Storey Rear Extensions
Detached House
- Up to 8m from rear wall
- Maximum 4m height
- Maximum 3m to eaves
Terraced/Semi-Detached
- Up to 6m from rear wall
- Maximum 4m height
- Maximum 3m to eaves
Important for London: Conservation areas, Article 4 directions, and listed building status remove PD rights. Many inner London boroughs have additional restrictions. Always check before assuming PD applies.
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Free, instant cost estimate based on your property details. Get professional 2026 pricing for your rear extension project.
Frequently Asked Questions
Do I need planning permission for a rear extension in 2026?
Not always. Single-storey rear extensions up to 3m (terraced/semi) or 6m (detached) may qualify for permitted development. However, conservation areas, listed buildings, and Article 4 directions in many London boroughs remove these rights. Most two-storey extensions need planning permission.
How much does a rear extension cost in London 2026?
Single-storey rear extensions typically cost £80k-100k on average in 2026, while two-storey extensions cost £140k-165k. Budget £3,200-3,700 per m² for basic spec, or £3,700-4,500 per m² for high spec finishes.
How long does a rear extension take to build?
Single-storey: 4-6 months build time. Two-storey: 6-9 months. Add 2-4 months for design and 8-12 weeks for planning if required. Total project duration is typically 8-14 months from initial consultation to completion.
Do I need party wall agreements?
Yes, if you are building on or within 3m of a party wall (shared boundary with neighbour). Most London terraced and semi-detached houses will need party wall agreements for rear extensions. Serve notices 2 months before work starts. Budget £1k-2.5k for party wall surveyor fees.
Can I live in my house during the extension?
Yes, most families do. Expect 2-4 weeks of major disruption when breaking through the rear wall. Your kitchen will be out of action during this time. Set up a temporary kitchen elsewhere. Dust and noise will be constant for 4-6 months.
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Summary
Rear extensions remain the most popular way to add space in London. For 2026, expect to pay £80k-100k for a single-storey extension or £140k-165k for two storeys. Costs have risen 3-4% from 2025, primarily due to labour inflation.
Start with accurate pricing and check your planning requirements before commissioning detailed designs. Our free tools can help you understand costs and planning constraints in under 2 minutes.