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 Barnes?
In Barnes, 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 |
What moves the price most:
- Roof design and tile matching
- Internal fit-out of the new rooms
- Single or two storeys
- Steel work where the side wall is opened
A guide range is only a starting point: the guide price tool gives an instant figure for your size, and a free survey turns it into a fixed quote.
Do you need planning permission for a side extension in Barnes?
Approvals for a side extension in Barnes 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.
Building regulations approval comes from the council’s building control service or a registered building control approver. The work is inspected at key stages, such as foundations, drainage and insulation, and a completion certificate is issued at the end.
At the survey we check your house’s planning history and any restrictions, and tell you which route applies.
What we handle
The work we manage on a side extension:
- Building control completion
- Foundations and drainage alterations
- Survey of the boundary, drains and access
- Planning drawings and application
- Walls, roof and windows to match the house
- Opening into the existing house
How long does it take?
The build itself usually takes 10 to 16 weeks on site depending on size and storeys. Before that, surveys, drawings and approvals usually take 3 to 4 months, longer if the council asks for changes.
Building in Barnes
Space is at a premium in London, so homeowners often build out into the side return or up into the roof rather than move.
From SW13 outwards, we take on projects throughout Barnes and the surrounding parts of Greater London.
How it works
- 01
Visit and measured survey
A free visit to understand the project, check access and take measurements.
- 02
Drawings and written quote
We prepare the design and give you a written, itemised quote with inclusions and exclusions.
- 03
Approvals
Planning or permitted development confirmation, building regulations and any party wall matters.
- 04
Build and handover
Work follows the agreed programme, with inspections along the way and a completion certificate at the end.
Questions about side extensions in Barnes
How close to the boundary can I build?
You can build up to the boundary, but the eaves height near it is limited under permitted development, and planning officers often want a gap at first floor. If you build on or near a shared boundary, party wall rules may apply.
How much does a side extension cost in Barnes?
In Barnes, 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.
Do you build side extensions in Barnes?
Yes. We build side extensions across Barnes, including postcodes SW13. Every project starts with a free home visit.
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.
How long does it take?
Roughly 10 to 16 weeks on site once approvals are in place, depending on whether it is one or two storeys.