Two-Storey Extension London: Complete Cost & Planning Guide
Add significant space with a two-storey extension. Everything you need to know about costs, planning permission, design, and building your double-storey extension in London.
Key Takeaways
- Average cost £130,000-150,000 in London (£2,500-3,500/m²)
- Most two-storey extensions need planning permission in London
- Total timeline: 4-6 months from planning to completion
- Get instant cost estimate for your project (Free)
Why Choose a Two-Storey Extension?
A two-storey extension doubles your space gain compared to a single-storey extension on the same footprint. While it costs more upfront, the cost per square metre is typically lower, making it excellent value if you need the space.
Single-Storey vs Two-Storey Comparison
Single-Storey (6m x 4m = 24m²)
- Cost: £60,000-84,000
- Cost per m²: £2,500-3,500
- Space gained: 24m²
Two-Storey (6m x 4m = 48m²)
- Cost: £130,000-150,000
- Cost per m²: £2,700-3,125
- Space gained: 48m²
Value comparison: Two-storey gives you double the space for roughly 2x the cost, making it more cost-effective per m².
Two-storey extensions work best when you need separate function spaces - typically kitchen/dining on the ground floor with bedroom and bathroom above. This is the most popular configuration in London.
Average Costs in London
| Cost Component | Budget Range | Premium Range |
|---|---|---|
| Construction (per m²) | £2,500-2,800 | £3,200-3,500 |
| Architect fees | £8,000-12,000 | £12,000-18,000 |
| Structural engineer | £2,000-3,500 | £3,500-5,000 |
| Planning application | £462 (fee) | £462 (fee) |
| Building control | £1,500-2,000 | £2,000-2,500 |
| Party wall surveyor | £1,200-2,000 | £2,000-3,000 |
| Total (typical 48m²) | £130,000-145,000 | £165,000-185,000 |
Budget factors:
- Ground conditions (clay soil increases foundation costs by 20-30%)
- Access restrictions (scaffolding from street adds 10-15%)
- Finish quality (kitchen, bathroom, flooring choices)
- Location within London (central zones typically 15-25% higher)
When Planning Permission is Required
Most two-storey rear extensions in London require planning permission. Permitted development rights for two-storey extensions are very restrictive (maximum 3m deep), which rarely provides enough space for a functional layout.
You ALWAYS Need Planning Permission If:
- Extension extends more than 3m from the rear wall
- Property is in a conservation area
- Property is listed (Grade I, II*, or II)
- Borough has Article 4 direction affecting two-storey extensions
- First floor wall would be less than 7m from rear boundary
- Previous owners have used up permitted development allowances
Planning Permission Timeline
- 1Pre-application advice: 4-6 weeks (optional but recommended)
- 2Application preparation: 2-4 weeks (drawings, documents)
- 3Statutory determination period: 8 weeks from validation
- 4If amendments required: Add 2-4 weeks
Total realistic timeline: 3-4 months from starting drawings to planning approval.
Design Considerations
Matching Existing Architecture
Planning officers expect two-storey extensions to match your existing house in style, materials, and proportions. This is crucial for planning approval.
Design Elements to Match
Roof Style
- Pitch angle (typically 35-45°)
- Roof covering (tiles/slates)
- Ridge height alignment
External Materials
- Brick type and colour
- Window frames (material/colour)
- Architectural details
Roof Design Options
The roof style significantly impacts both cost and planning approval likelihood:
Pitched Roof (Most Common)
Matches existing house, best for planning approval. Allows full-height first floor ceiling. Cost: standard.
Best for: Traditional terraced/semi-detached houses, conservation areas
Flat Roof (Modern Option)
Lower profile, can be more economical. May face planning scrutiny in traditional areas. Cost: 10-15% less than pitched.
Best for: Modern properties, when matching existing flat roof
Hybrid (Pitched over Flat)
Pitched roof on front elevation, flat to rear. Good compromise for planning. Cost: mid-range.
Best for: Maximising internal space while satisfying planning requirements
Window Placement
First floor windows require careful consideration to avoid neighbour objections:
- Rear-facing windows are generally acceptable (overlooking your own garden)
- Side-facing windows overlooking neighbours often require obscure glazing
- High-level windows (1.7m+ sill height) can avoid privacy issues
- Align window sizes and positions with existing house for visual harmony
Structural Requirements
Foundations
Two-storey extensions require deep strip or trench fill foundations because of the increased load. Depth typically 1.0-1.2m (deeper in clay soils).
Foundation Cost Factors
- Ground conditions: Clay soil requires deeper foundations (add £3,000-5,000)
- Trees nearby: May require deeper foundations or piling (add £5,000-15,000)
- Access: Poor access increases excavation costs by 15-25%
- Waste disposal: Budget £800-1,500 for soil removal
Steelwork
Structural steel beams support loads and create open-plan spaces. Two-storey extensions typically require:
- Ground floor: 203x133mm UB or 254x146mm UB (supporting first floor)
- First floor: 178x102mm UB (supporting roof)
- Padstones at bearing points (minimum 215mm solid wall)
- Budget £3,000-6,000 for steelwork supply and installation
Critical: All structural calculations must be certified by a chartered structural engineer and approved by building control before installation.
Common Layouts
Most Popular: Kitchen-Dining Below, Bedroom-Bathroom Above
Ground Floor (6m x 4m = 24m²)
- Open-plan kitchen and dining area
- Large glazing to garden (bifold/sliding doors)
- Central island (if width allows, min 3.5m)
- Connection to existing living room
First Floor (6m x 4m = 24m²)
- Master bedroom (typically 4m x 3.5m = 14m²)
- En-suite bathroom (typically 2.5m x 2m = 5m²)
- Built-in wardrobes along one wall
- Access from existing landing
Alternative layouts: Study + bedroom, two smaller bedrooms, bedroom + walk-in wardrobe.
Layout tip: Position the bathroom above the kitchen to simplify plumbing runs and reduce costs by £1,000-2,000.
Timeline: 4-6 Months Total
| Phase | Duration | Key Activities |
|---|---|---|
| Design & Planning | 8-12 weeks | Architect drawings, planning application |
| Building Regs | 4-6 weeks | Structural calculations, building control approval |
| Party Wall | 2-4 weeks | Serve notices, neighbour agreement (runs parallel) |
| Construction | 12-16 weeks | Foundations to completion |
| Total | 4-6 months | From starting design to moving in |
Construction Phase Breakdown
- 1Weeks 1-2: Groundworks and foundations
- 2Weeks 3-6: Ground floor structure and first floor build
- 3Weeks 7-8: Roof construction and weatherproofing
- 4Weeks 9-10: Windows, doors, first fix (electrics, plumbing)
- 5Weeks 11-12: Plastering and second fix
- 6Weeks 13-16: Kitchen, bathroom fit-out, decorating, snagging
Party Wall Requirements
If you share a wall with a neighbour (terraced or semi-detached), you must comply with the Party Wall Act 1996.
When Party Wall Notice Required
- Building on or up to the party wall line
- Cutting into or raising the party wall
- Excavating within 3m of neighbour's structure (and deeper than their foundations)
- Excavating within 6m (if you'll dig deeper than their foundations minus 45° line)
Party Wall Process
- 1Serve notice: At least 2 months before construction starts (use registered post)
- 2Neighbour responds: They have 14 days to consent or dissent
- 3If consent: Work can proceed (after 2 month notice period)
- 4If dissent: Both parties appoint surveyors who draft a Party Wall Award
- 5Award sets terms: Schedule of condition, permitted work, access arrangements
Party Wall Costs
- Your surveyor: £1,200-2,000 (you always pay this)
- Neighbour's surveyor: £1,200-2,000 (you pay if they dissent)
- Total if neighbour agrees: £1,200-2,000
- Total if neighbour dissents: £2,400-4,000
Pro tip: Talk to your neighbours before serving formal notice. Explain your plans, show them drawings, and address concerns early. Most neighbours consent when they understand the project.
Building Regulations Checklist
All extensions require building regulations approval, even if planning permission isn't needed. Here's what building control will check:
Structural Safety
- Foundation depth and design
- Steel beam specifications
- Wall tie and lintel design
- Roof structure and loading
Fire Safety
- 30-min fire doors to kitchen
- Fire-rated ceiling below stairs
- Smoke alarms (interlinked)
- Escape window from bedroom
Insulation & Energy
- Wall U-value 0.18 W/m²K
- Roof U-value 0.15 W/m²K
- Floor U-value 0.18 W/m²K
- Windows U-value 1.4 W/m²K
Drainage & Services
- Foul drainage connections
- Rainwater drainage
- Ventilation rates (kitchen/bath)
- Electrical certification
Inspection schedule: Building control typically inspects at 5 stages - foundations, DPC, first floor joists, roof structure, and final completion. Allow 48 hours notice for each inspection.
Planning Permission Tips
Increasing Your Approval Chances
Match existing materials exactly
Use the same brick type, mortar colour, roof tiles, and window styles as your existing house. Submit material samples if required by the planning officer.
Keep below the ridge line
Extension roof should finish below the main house ridge. This makes the extension appear subordinate to the original house - a key planning principle.
Minimize neighbour impact
Keep first floor walls at least 7m from boundaries where possible. Use a 45° daylight test to prove you're not blocking significant light to neighbour windows.
Reference approved neighbours
If neighbours have similar extensions, reference those planning applications. Search your council's planning portal and cite similar approved schemes in your design statement.
Pre-application advice
Consider paying £150-300 for pre-application advice from your council. This gives you direct feedback from the planning officer before submitting, reducing rejection risk.
Common Rejection Reasons
- Excessive bulk/massing: Extension too large for the plot
- Loss of light: Significant impact on neighbour's windows
- Overlooking concerns: Privacy impact from first floor windows
- Poor design quality: Doesn't match existing house character
- Conservation area: Harm to character of area
Get Your Free Extension Estimate
Our Free Cost Estimator provides accurate pricing for your two-storey extension based on:
- Your exact location and local build costs
- Extension size and configuration
- Finish quality preferences
- Professional fees and consents
Frequently Asked Questions
Is a two-storey extension cheaper per m² than two single-storey extensions?
Yes, typically 15-20% cheaper per m². You share the foundation, external walls, and roof costs across both floors. Two separate single-storey extensions would require duplicating these elements, making them less cost-effective.
Can I live in my house during construction?
Usually yes, but expect significant disruption during weeks 1-8 (groundworks and structure). You'll lose kitchen access during weeks 10-16. Many families move out for 2-3 months or set up a temporary kitchen. Budget £2,000-4,000 if renting temporarily.
Do I need an architect for a two-storey extension?
Highly recommended. Architects from organizations like RIBA understand planning policies, design constraints, and how to maximize space. Their fee (6-10% of build cost) is usually recouped through better design, fewer planning issues, and avoiding costly mistakes. Expect to pay £8,000-18,000 depending on complexity.
How much value does a two-storey extension add?
In London, extensions typically add £2,000-3,000 per m² to property value. A 48m² two-storey extension could add £96,000-144,000 to your home value, compared to a £130,000-150,000 build cost. This gives excellent ROI, especially in high-demand areas.
What if my neighbour objects to planning?
Neighbour objections are considered but don't automatically result in refusal. Planning officers assess on planning grounds (light loss, privacy, design) not personal preferences. Address legitimate concerns in your design. If refused, you can appeal or submit revised plans within 12 months (no additional fee).
Can I add a third storey instead?
Technically possible but faces significant planning hurdles. Third storeys often fail due to excessive height, overlooking, and being out of character with the area. They're also structurally more complex (requiring foundation upgrades to the entire house) and typically cost £3,500-4,500/m². Stick with two storeys unless you have exceptional planning circumstances.
What heating system do I need for the extension?
If your existing boiler has 25-30% spare capacity, you can extend the system (budget £2,500-4,000 for radiators and pipework). Otherwise, budget £3,000-5,000 for a new A-rated combi boiler. Underfloor heating on the ground floor is popular (adds £60-80/m²) and works well with the high-quality insulation required by building regulations.
Summary
Two-storey extensions offer exceptional value for money in London if you need the space. While they cost more upfront than single-storey extensions (£130,000-150,000 vs £60,000-84,000), you get double the space for roughly double the price, making them more cost-effective per square metre.
Most two-storey extensions require planning permission because permitted development limits are too restrictive (3m depth maximum). Budget 3-4 months for planning and consents, then 3-4 months for construction. Total project timeline: 4-6 months.
Key success factors: Design that matches your existing house, good neighbour relations, experienced professionals (architect and builder), and realistic budget with 15% contingency for unforeseen issues.