Semi-Detached House Extensions London 2025: Complete Guide
The definitive guide to extending your semi-detached home in London. From rear and side extensions to loft conversions and wrap-arounds, understand your options, costs, and planning requirements.
Key Takeaways
- Single-storey rear extensions up to 6m under permitted development (4m if attached side)
- Only one shared party wall (easier than terraced houses)
- Side extension potential on detached side creates unique opportunities
- Get instant cost estimate for your semi-detached extension (Free)
Why Semi-Detached Houses Are Different
Semi-detached houses sit in a sweet spot for extensions. You have one shared party wall (unlike terraced homes with two), and typically more side space than terraced properties. This creates unique opportunities:
- Simpler party wall process (only one neighbour to coordinate with)
- Potential for side extensions on the detached side
- More design flexibility than terraced homes
- Option for wrap-around extensions combining rear and side
Rear Extensions for Semi-Detached Houses
Single-Storey Rear Extensions
The most popular extension type for semi-detached homes. Your permitted development limits depend on which side is attached:
Attached Side
- Maximum 6m from rear wall
- Up to 4m if extended 3-6m
- Requires Prior Approval 3-6m
Detached Side
- Maximum 8m from rear wall
- Up to 4m if extended 4-8m
- Requires Prior Approval 4-8m
Key point: Most semi-detached homes are treated as having an attached side for PD purposes, limiting single-storey rear extensions to 6m unless you have exceptional separation.
Typical Costs: Single-Storey Rear
| Extension Size | Cost Range | per m² |
|---|---|---|
| 3m x 4m (12m²) | £36,000 - £48,000 | £3,000 - £4,000 |
| 4m x 6m (24m²) | £60,000 - £84,000 | £2,500 - £3,500 |
| 6m x 6m (36m²) | £90,000 - £126,000 | £2,500 - £3,500 |
Two-Storey Rear Extensions
Two-storey rear extensions on semi-detached homes are more restricted under permitted development:
- Maximum 3m from rear wall (all house types)
- Cannot extend beyond the rear of the attached side wall
- First floor must be at least 7m from rear boundary
Reality check: Most semi-detached extensions deeper than 3m two-storey require planning permission. However, planning approval rates for well-designed two-storey extensions are high (70-80%).
Typical Costs: Two-Storey Rear
| Extension Size | Cost Range | per m² |
|---|---|---|
| 3m x 4m (24m² total) | £60,000 - £84,000 | £2,500 - £3,500 |
| 4m x 6m (48m² total) | £110,000 - £165,000 | £2,300 - £3,400 |
| 6m x 6m (72m² total) | £165,000 - £245,000 | £2,300 - £3,400 |
Side Extensions on Semi-Detached Houses
This is where semi-detached houses shine. The detached side offers extension potential that terraced homes don't have.
Permitted Development Limits
- Maximum width: half the width of the original house
- Single storey only (maximum 4m height)
- Must be at least 7m from opposite boundary
- No closer than 2m from front property line
Common issue: Many semi-detached homes have less than 7m width between the house and side boundary, meaning side extensions require planning permission. However, approval rates are typically high if you maintain 1m+ from the boundary.
Typical Costs: Side Extensions
| Extension Size | Cost Range |
|---|---|
| 2m x 6m (12m²) | £36,000 - £48,000 |
| 3m x 8m (24m²) | £60,000 - £84,000 |
Wrap-Around Extensions
Combining rear and side extensions creates a wrap-around extension. This is one of the most popular options for semi-detached homes because it:
- Maximizes ground floor space (typically 30-50m² added)
- Creates large open-plan kitchen/dining/living area
- Provides corner glazing opportunities for more light
- Often provides the best value per m² added
Planning Considerations
Wrap-around extensions almost always require planning permission because:
- Side element usually doesn't meet 7m boundary rule
- Combined volume may exceed PD allowances
- May exceed half-width rule for side extensions
However, approval rates are typically 75-85% for well-designed wrap-arounds that respect neighbour amenity and match existing materials.
Typical Costs: Wrap-Around Extensions
| Extension Size | Cost Range | per m² |
|---|---|---|
| 30m² total | £75,000 - £105,000 | £2,500 - £3,500 |
| 40m² total | £100,000 - £140,000 | £2,500 - £3,500 |
| 50m² total | £125,000 - £175,000 | £2,500 - £3,500 |
Loft Conversions
Loft conversions work particularly well on semi-detached homes because you can often add a rear dormer on the detached side without affecting neighbours.
Permitted Development Limits
- Volume increase up to 50m³ (semi-detached)
- Rear dormer must be 20cm from eaves and party wall
- No extension beyond plane of existing roof slope facing highway
- Materials must be similar to existing house
Advantage: The party wall only applies if you're working within 3m of the shared boundary, so roof work on the detached side may not trigger party wall requirements.
Typical Costs: Loft Conversions
| Conversion Type | Cost Range |
|---|---|
| Velux (roof windows only) | £25,000 - £35,000 |
| Rear dormer | £35,000 - £50,000 |
| Hip-to-gable with dormer | £45,000 - £65,000 |
Party Wall Act: Simplified for Semi-Detached
Semi-detached homes have a major advantage over terraced homes: only one shared wall. This simplifies the party wall process significantly:
Terraced House
- Two party walls to manage
- Two sets of notices required
- Higher surveyor costs (£1,500-2,500)
Semi-Detached House
- One party wall only
- One set of notices
- Lower surveyor costs (£800-1,500)
When Party Wall Applies
You need party wall notice if:
- Working directly on the shared wall (foundations, structural changes)
- Excavating within 3m of the party wall (foundations for extensions)
- Excavating within 6m and deeper than neighbour's foundations
Timeline: Serve notice 2 months before construction. If neighbour agrees, can start after 14 days. Budget 2-3 months for the full process if appointing surveyors.
Design Considerations for Semi-Detached Extensions
Symmetry with Neighbour
One unique consideration for semi-detached homes is visual symmetry with your attached neighbour. Planning officers often prefer designs that:
- Maintain the rhythm of windows with the existing pair
- Match roofline if extending two-storey
- Use matching materials (especially for front or side elevations)
- Keep proportions similar to original house
Privacy and Overlooking
Key privacy considerations:
- Avoid windows facing directly into neighbour's windows (especially on party wall side)
- Use obscure glazing or high-level windows on side elevations
- Maintain 21m between rear windows and opposite properties
- Consider impact of two-storey rear extensions on neighbour's garden
Natural Light
Planning officers assess impact on neighbour's light using the 45-degree rule:
- Draw 45° line from center of neighbour's nearest window in plan
- Draw 45° line from windowsill in elevation
- Extension should not breach these lines
When Planning Permission Is Required
You'll need planning permission for semi-detached extensions if:
- Exceeding permitted development size limits
- Property in conservation area (restricted PD)
- Listed building (need planning + listed building consent)
- Article 4 direction in place
- Side extension closer than 7m from opposite boundary
- Two-storey extension deeper than 3m
- Wrap-around extension (almost always)
- Previous PD rights exhausted
Planning Timeline and Costs
| Application Type | Timeline | Council Fee |
|---|---|---|
| Prior Approval | 6 weeks | £96 |
| Householder Planning | 8 weeks | £258 |
| Listed Building Consent | 8 weeks | £150 |
Common Challenges and Solutions
Challenge: Narrow Side Access
Many semi-detached homes have less than 7m between house and side boundary, preventing PD for side extensions.
Solution: Apply for planning permission. Most councils approve if you maintain 1m+ from boundary and design complements the existing house. Consider submitting a pre-application for feedback.
Challenge: Matching Materials
Original bricks may be discontinued or expensive to source exactly.
Solution: Work with a brick matching service to find closest match, or consider a complementary material (like dark brick with red original) that creates intentional contrast rather than failed match.
Challenge: Party Wall Neighbour Dispute
Neighbour may refuse to agree to party wall notice or be difficult about access.
Solution: Appoint party wall surveyors (one for you, one for them, or one agreed surveyor). This adds £800-1,500 cost but ensures proper process. Surveyors often defuse tension by providing neutral third-party oversight.
Challenge: Maintaining Symmetry
Planning may require design changes to maintain visual symmetry with attached neighbour.
Solution: Engage an architect early who understands local planning policy. They can design extensions that work with the pair rather than against it, often adding to street character rather than detracting.
Get Your Free Extension Estimate
Our instant estimate tool provides accurate costs for your semi-detached extension:
- Rear, side, wrap-around, and loft conversion costs
- Party wall and planning costs included
- London-specific pricing
- Instant results, no signup required
Frequently Asked Questions
How much value does an extension add to a semi-detached house?
Well-designed extensions typically add 15-20% to property value. A 30m² extension costing £90,000 on a £500,000 home should add £75,000-£100,000 in value. Kitchen/dining extensions and additional bedrooms add the most value.
Can I extend right up to the party wall?
Yes, you can build up to the party wall line, but you cannot build on or over it without neighbour agreement. The Party Wall Act covers the process for work within 3m of the shared wall. Most rear extensions on semi-detached homes do extend to the party wall line.
Do I need my neighbour's permission to extend?
No. Your neighbour cannot veto your extension. However, you must serve party wall notice for work affecting shared structures, and they can object during the planning consultation period if you need planning permission. It's always better to maintain good relations by discussing plans early.
What if my neighbour has already extended?
If your neighbour has extended, it often helps your planning case because you're maintaining symmetry with the pair. However, their extension doesn't automatically grant you permission. Each application is assessed on its own merits.
Should I do a wrap-around or separate rear and side extensions?
Wrap-arounds are usually more cost-effective (single build process, one set of fees) and create better internal flow. Separate extensions might be suitable if phasing construction over multiple years or if one element qualifies for PD while the other needs planning permission.
Can I convert my garage and extend in the same project?
Yes, this is common for semi-detached homes with side garages. Converting the garage and adding a rear extension creates a large ground floor living space. Garage conversions are often PD if you maintain the existing opening, though most people prefer to replace the garage door with matching brickwork and windows (requires planning).
Summary
Semi-detached houses offer excellent extension potential in London. With only one shared party wall and opportunities for side extensions, you have more flexibility than terraced homes while enjoying similar urban density benefits.
Whether you're planning a simple rear extension, ambitious wrap-around, or loft conversion, the key to success is:
- Understanding your permitted development rights early
- Managing the party wall process professionally
- Designing with symmetry and context in mind
- Getting accurate cost estimates before committing to design fees
Start with our free tools to understand your options and budget, then engage professionals to bring your vision to life.