What is a side extension?
A side extension builds into the space beside a semi-detached or detached house, often over a driveway or side passage. It can be single-storey, for a utility room, downstairs toilet or study, or two-storey to add a bedroom above.
How much does a side extension cost in Balham?
In Balham, a side extension typically costs £2,200–£3,450 per m² of floor area, including VAT at 20% (standard rate). For a typical 28 m² project that is roughly £61,500–£96,500. That range is adjusted for London, where domestic building work typically costs around 30% above the national average. Design, structural calculations and council fees are extra.
| Size or type | Guide range |
|---|---|
| Smaller project (20 m²) | £44,000–£69,000 |
| Typical project (28 m²) | £61,500–£96,500 |
| Larger project (39 m²) | £86,000–£134,500 |
| Rate per m² | £2,200–£3,450 |
Your quote will depend mainly on:
- Drains, gas meters or soil pipes that need moving
- Internal fit-out of the new rooms
- Boundary walls and access for materials
- Single or two storeys
Use the guide price tool to see how size and specification change the figure, then ask for a written, itemised quote.
Do you need planning permission for a side extension in Balham?
Approvals for a side extension in Balham follow the rules for England. In England a single-storey side extension can be permitted development if it is no more than 4 metres high and no wider than half the width of the original house. Side extensions of more than one storey always need planning permission, as do side extensions in conservation areas, National Parks and National Landscapes.
Where work involves a shared wall, building on the boundary, or digging foundations within 3 metres of a neighbour’s building and deeper than its foundations (or within 6 metres for deeper work), the Party Wall etc. Act 1996 applies and notices must be served before work starts.
We check this for your address before design work starts, so there are no surprises later.
What we handle
What we take care of:
- Opening into the existing house
- Building control completion
- Foundations and drainage alterations
- Walls, roof and windows to match the house
- Survey of the boundary, drains and access
- Structural calculations
How long does it take?
Expect 10 to 16 weeks on site depending on size and storeys. Design and approvals come first and typically take 3 to 4 months, depending on the approval route.
Building in Balham
Victorian and Edwardian terraces dominate many London streets, which is why side return, wraparound and mansard projects are so common here.
If your home is in Balham or close by in Clapham, Streatham and Tooting, a free home visit is the first step.
How it works
- 01
Visit and measured survey
A free visit to understand the project, check access and take measurements.
- 02
Design and fixed quote
Drawings, structural calculations and an itemised, fixed price for the agreed specification.
- 03
Approvals
Planning or permitted development confirmation, building regulations and any party wall matters.
- 04
Build and sign-off
We run the build to a written programme and hand over the completion certificate.
Questions about side extensions in Balham
Can a side extension go over a drain?
Often yes, with a build-over agreement from the water company or by rerouting the drain. We check this during the survey.
Will a two-storey side extension look odd?
Not if it is designed well. Setting it back from the front and keeping the roof lower than the main ridge usually gives a natural look and is what most councils ask for.
Can I build a side extension without planning permission?
In England a single-storey side extension can be permitted development if it is no more than 4 metres high and no wider than half the original house, and the house is not in a conservation area or similar protected land. Two-storey side extensions always need planning permission.
How long does a side extension take in Balham?
Typically 10 to 16 weeks on site depending on size and storeys. Design and approvals beforehand usually take 3 to 4 months.
How much does a side extension cost in Balham?
In Balham, a side extension typically costs £2,200–£3,450 per m² of floor area, including VAT at 20% (standard rate). A typical 28 m² project comes to roughly £61,500–£96,500. Your fixed price depends on the survey and drawings.