Rear Extension London 2025: Complete Cost & Planning Guide
Everything you need to know about rear extensions in London. From costs and planning requirements to design options and permitted development limits.
Key Takeaways
- Single-storey rear extensions cost £75k-90k on average in London
- Two-storey rear extensions cost £130k-150k on average
- Build 3-6m under permitted development depending on house type
- Get instant cost estimate (Free)
Single vs Two-Storey Rear Extensions
The choice between single and two-storey rear extensions is one of the most important decisions you'll make. It affects cost, planning, and how you use your home.
Single-Storey
- £75k-90k average cost
- May qualify for PD
- 4-6 month build time
- Ideal for kitchen-diners
Two-Storey
- £130k-150k average cost
- Usually needs planning
- 6-9 month build time
- Adds bedroom + living space
Rear Extension Costs in London (2025)
Average Total Costs
London rear extension costs vary significantly based on size, specification, and location:
| Extension Type | Size | Average Cost |
|---|---|---|
| Small single-storey | 15-20m² | £65k-75k |
| Medium single-storey | 20-30m² | £75k-90k |
| Large single-storey | 30-40m² | £90k-110k |
| Small two-storey | 30-40m² | £110k-130k |
| Medium two-storey | 40-50m² | £130k-150k |
| Large two-storey | 50-60m² | £150k-180k |
Cost Per Square Metre Breakdown
London rear extensions typically cost £3,000-3,500 per m² for basic spec, rising to £3,500-4,500 per m² for high spec.
What's 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: £4k-8k (8-12% of build cost)
- Structural engineer: £1.5k-3k
- Party wall agreements: £1k-2k (if applicable)
- Planning fees: £258-462 (if required)
- Kitchen: £8k-25k+ depending on spec
- Contingency: 10-15% of total budget
Permitted 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: Extensions 3-4m (terraced/semi) or 6-8m (detached) deep require Prior Approval for neighbour notification (6 week process, £96 fee).
Two-Storey Rear Extensions
Two-storey rear extensions have much more restrictive PD limits:
- Maximum 3m from rear wall (all house types)
- No extension beyond the rear of the side wall
- First floor rear wall must be at least 7m from boundary
- Maximum height not exceeding original ridge height
Reality check: Most two-storey rear extensions in London exceed PD limits and require planning permission. The 3m limit is too restrictive for most family needs.
When You Need Planning Permission
You'll need planning permission for your rear extension if:
- Your extension exceeds PD size limits
- Your property is in a conservation area
- Your property is listed
- Your borough has Article 4 directions removing PD rights
- Previous extensions have used up your PD allowance
- You want to add windows in side walls facing boundaries
Planning Permission Timeline & Cost
- Timeline: 8 weeks statutory period (often extends to 10-12 weeks)
- Application fee: £258 (householder application)
- Success rate: 85-90% for rear extensions with good design
Roof Design: Flat vs Pitched
Flat Roof
Most popular in London for single-storey extensions.
Advantages
- Modern, contemporary look
- Can add rooflights easily
- Lower overall height
- Slightly lower cost
Disadvantages
- Needs regular maintenance
- May not suit traditional homes
Pitched Roof
Better for matching existing house character.
Advantages
- Matches traditional houses
- Better for conservation areas
- Less maintenance long-term
- Better drainage
Disadvantages
- Higher overall height
- May exceed PD height limits
- Slightly higher cost
Bi-fold Doors and Glazing Options
Large glazed openings are essential for modern rear extensions, connecting inside and outside while flooding your new space with natural light.
Bi-fold Doors
- Cost: £2,500-4,000 for a 3-4m wide opening (mid-spec aluminium)
- Most popular: 3-panel or 4-panel configurations
- Material: Aluminium is best for London (low maintenance, slim frames)
- Popular brands: Origin, Schuco, Sunflex
Sliding Doors
Increasingly popular as an alternative to bi-folds:
- Better thermal performance than bi-folds
- Cleaner sightlines (larger glass panels)
- Cost: £3,000-5,000 for premium lift-and-slide systems
- No panels folding back (saves space outside)
Rooflights
Essential for bringing light deep into your extension:
- Standard rooflights: £500-800 each (e.g., VELUX)
- Structural glass rooflights: £2,000-4,000 (frameless, flush with ceiling)
- Lantern roofs: £3,000-6,000 (larger glazed roof structures)
Kitchen-Diner Layouts
The open-plan kitchen-diner is the primary reason most people build rear extensions. Here's how to get the layout right:
Typical Layout Options
L-Shaped Kitchen
Most popular for rear extensions. Kitchen along two walls, with dining/living in the extension.
- Best for terraced/semi-detached houses
- Efficient workflow triangle
Island Kitchen
Kitchen along one wall, with island unit separating cooking and dining areas.
- Requires 4m+ width for good circulation
- Great for entertaining and family life
Galley Kitchen
Kitchen units along both long walls. Less common for extensions but efficient for narrow spaces.
- Best for narrower extensions
- Very efficient workflow
Design Tips
- Position dining table near bi-folds for indoor-outdoor living
- Aim for 1.2m+ clear circulation space around islands
- Consider underfloor heating throughout (adds £50-70 per m²)
- Plan for plenty of electrical sockets (islands need floor boxes)
- Use different floor finishes to subtly zone cooking and dining areas
Timeline and Process
A typical rear extension project takes 8-14 months from start to finish. Here's the breakdown:
| Stage | Duration | Key Activities |
|---|---|---|
| Design | 4-8 weeks | Architect drawings, structural calculations |
| Planning | 8-12 weeks | Submit application, wait for decision (if required) |
| Party wall | 4-8 weeks | Notices, neighbour agreements (if applicable) |
| Tendering | 2-4 weeks | Get quotes, select builder |
| Construction | 4-6 months (single) 6-9 months (double) | Build, fit-out, finishing |
| Completion | 1-2 weeks | Snagging, building control sign-off |
Pro tip: You can overlap stages. Start design while planning application is pending. Get party wall notices out early. This can save 2-3 months on the overall timeline.
Foundations and Structural Work
The foundation type significantly affects cost and timeline. Your structural engineer will specify based on ground conditions and existing building.
Foundation Types
Strip Foundations
Most common for single-storey extensions. Concrete strips under load-bearing walls.
- Cost: £60-100 per m² of floor area
- Typical depth: 1-1.5m
Raft/Slab Foundations
Concrete slab under entire extension. Used on difficult ground or where strip foundations aren't suitable.
- Cost: £100-150 per m² of floor area
- Faster installation than strip
Piled Foundations
Required for very poor ground or where trees affect soil. Concrete piles driven deep into ground.
- Cost: £150-250+ per m² of floor area
- Common in areas with clay soil and mature trees
Steel Beams
Opening up your existing rear wall requires structural steel beams to support the load above:
- Single-storey load: £1,500-3,000 per beam span
- Two-storey load: £3,000-5,000+ per beam span
- Typically need temporary supports (Acrow props) during installation
Building Regulations Checklist
You always need building regulations approval for rear extensions, even if you're using permitted development. Here's what building control will check:
Structural Stability
Foundations, walls, roof structure must meet load-bearing requirements
Fire Safety
Escape routes, fire-rated doors, smoke alarms. Critical for two-storey extensions.
Thermal Performance
U-values for walls, roof, glazing. SAP calculations for energy efficiency.
Damp Proofing
DPC, DPM, drainage, ventilation to prevent moisture problems.
Ventilation
Extract fans, background ventilation, air circulation standards.
Drainage
Foul drainage connections, surface water management, soakaway requirements.
Electrical Safety
Part P compliance, certified electrician, consumer unit upgrades if needed.
Glazing and Guarding
Safety glass where required, balustrades if level changes exceed 600mm.
Building Control Costs & Timing
- Fees: £800-1,200 typical for rear extension
- Inspections: 4-6 site visits during construction
- Completion certificate: Issued after final inspection passes
Get Your Free Extension Estimate
Answer a few questions about your property and get an instant, detailed cost estimate for your rear extension project.
- Instant cost breakdown by trade and material
- Professional fees and timeline estimate
- Planning and PD assessment
- Detailed PDF report to share with contractors
Frequently Asked Questions
Do I need planning permission for a rear extension?
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?
Single-storey rear extensions typically cost £75k-90k on average, while two-storey extensions cost £130k-150k. Budget £3,000-3,500 per m² for basic spec, or £3,500-4,500 per m² for high spec finishes. Add 10-15% contingency plus professional fees.
How long does a rear extension take to build?
Single-storey rear extensions take 4-6 months to build. Two-storey extensions take 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.
Should I choose bi-fold doors or sliding doors?
Bi-fold doors (£2,500-4,000) are the traditional choice with good ventilation when fully open. Sliding doors (£3,000-5,000) offer better thermal performance, cleaner sightlines, and don't need external clearance space. Both work well - choose based on your budget and aesthetic preference.
Do I need party wall agreements?
Yes, if you're 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-2k for party wall surveyor fees.
What's the best time of year to build an extension?
Spring and summer are ideal - better weather speeds up groundworks and external work. However, good builders book up 3-6 months ahead. Start your planning and design in autumn/winter to be ready for a spring start. Avoid starting major works in November-February if possible.
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, so consider if young children or remote workers can cope.
Should I extend or move house?
If you love your location and need 20-40m² more space, extending is usually better value than moving. Stamp duty, estate agent fees, and moving costs often exceed extension costs. However, if you need more bedrooms or fundamentally different space, moving might make more sense. Get a cost estimate to compare properly.
Summary
Rear extensions are the most popular way to add space to London homes. They deliver excellent value - creating modern, light-filled spaces that transform how you live.
Budget £75k-90k for single-storey or £130k-150k for two-storey. Check your permitted development rights early to understand if you need planning permission.
The process takes 8-14 months from start to finish, but the result is a transformed home that better suits your family's needs and adds significant value to your property.