What is a garden annexe?
A garden annexe is a single-storey building in the garden with its own living space, bathroom and often a kitchenette, used by relatives, older children or guests. Unlike a garden room, it is built as habitable accommodation with foundations, insulation and services.
How much does a garden annexe cost in Barnet?
In Barnet, a garden annexe typically costs £2,350–£3,650 per m² of floor area, including VAT at 20% (standard rate). For a typical 35 m² project that is roughly £82,000–£128,000. Building costs in London run about 30% above the UK average, and the figures for Barnet are adjusted to reflect that. Design, structural calculations and council fees are extra.
| Size or type | Guide range |
|---|---|
| Smaller project (24 m²) | £56,500–£87,500 |
| Typical project (35 m²) | £82,000–£128,000 |
| Larger project (49 m²) | £115,000–£179,000 |
| Rate per m² | £2,350–£3,650 |
The main things that change the cost:
- Heating type
- Cladding and finishes
- Drainage connection distance
- Kitchen and bathroom
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 garden annexe in Barnet?
The approval route for Barnet is set by planning law in England. In England an annexe used as living accommodation usually needs planning permission. Outbuildings built under permitted development must be incidental to the house, such as a gym or office, not a bedroom.
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.
If the answer is not clear-cut, we can apply for formal confirmation before you commit to the build.
What we handle
What we take care of:
- External cladding or brickwork
- Kitchenette and shower room
- Insulated walls and roof
- Glazed doors and windows
- Building control sign-off
- Foundations and drainage connections
How long does it take?
Expect 10 to 16 weeks on site. Allow time beforehand for drawings and approvals, usually 3 to 4 months.
Building in Barnet
Space is at a premium in London, so homeowners often build out into the side return or up into the roof rather than move.
Our Barnet coverage includes postcode districts EN5, and we also work in nearby places such as Potters Bar, Mill Hill and Borehamwood.
How it works
- 01
Free visit
We look at the property, talk through what you want and measure up. There is no charge and no obligation.
- 02
Design and fixed quote
Drawings, structural calculations and an itemised, fixed price for the agreed specification.
- 03
Approvals in place
We handle planning, building control and neighbour notices before work starts.
- 04
Construction and completion
We build, put right any snags and hand over the building control certificate.
Questions about garden annexes in Barnet
Does it need building regulations?
Yes, as it contains sleeping accommodation and services.
How long does it take?
Typically 10 to 16 weeks on site.
Who approves a garden annexe in Barnet?
Planning matters in Barnet go to the council covering your address, and the work needs building control approval. We prepare and submit the applications for you.
Can an annexe be rented out?
Not without further planning permission, as that would make it a separate dwelling.
How much does a garden annexe cost in Barnet?
In Barnet, a garden annexe typically costs £2,350–£3,650 per m² of floor area, including VAT at 20% (standard rate). A typical 35 m² project comes to roughly £82,000–£128,000. Your fixed price depends on the survey and drawings.