Garden rooms and outbuildings in Barnet depend on the house. Typical homes here: 1930s semi-detached houses in Finchley, Hendon, Edgware, Mill Hill and Whetstone; Arts and Crafts and neo-Georgian houses in Hampstead Garden Suburb. Designations to check first: Hampstead Garden Suburb: Article 4 direction and Trust consent; Conservation areas; Green Belt. Applications are decided by London Borough of Barnet and the Barnet Local Plan.
The permitted development rules for garden rooms and outbuildings
An outbuilding is permitted development on a house if it is single storey, no higher than 2.5 m where it comes within 2 m of a boundary, otherwise no higher than 4 m with a dual-pitched roof or 3 m with any other roof, sits behind the front wall of the house, covers together with other outbuildings no more than half the land around the original house, and is used for purposes incidental to the house rather than as a bedroom or self-contained flat.
Anything used for sleeping or as a separate dwelling needs planning permission regardless of size. Flats have no permitted development rights for outbuildings, and listed buildings need listed building consent.
What changes in Barnet
In a conservation area an outbuilding is still permitted development behind the house, but not to the side of it, and the same height limits apply.
The local constraints that most often decide the route in Barnet are listed below, taken from our Barnet area page. Each one is a reason to confirm the address on Barnet's constraints map before relying on permitted development.
- Hampstead Garden Suburb: Article 4 direction and Trust consent. Hampstead Garden Suburb is a conservation area with a long-standing Article 4 direction that removes most householder permitted development rights, so almost any external change needs planning permission. Source
- Conservation areas. Barnet has numerous conservation areas, including Hampstead Garden Suburb, Monken Hadley, Totteridge, Mill Hill, Finchley Church End and Hendon (The Burroughs). Source
- Green Belt. Large tracts of Totteridge, Arkley, Mill Hill and the northern fringe of the borough are Green Belt. Source
- Other Article 4 directions and HMOs. Beyond Hampstead Garden Suburb, Barnet may have Article 4 directions affecting other conservation areas and houses in multiple occupation. Source
What Barnet planning officers look for
The most common refusal reasons we encounter are two-storey side extensions that are not set back from the front elevation and create a terracing effect between semis, rear extensions deeper than the neighbour's that fail the 45-degree test, and front dormers. Hip-to-gable lofts are normally acceptable outside conservation areas. Barnet offers a paid pre-application service, worth using in Hampstead Garden Suburb and the Green Belt. Decision times often exceed the statutory 8 weeks; a written extension of time is common.
Fees and timelines
Barnet decides a householder application or a Lawful Development Certificate within 8 weeks of validation. The council fee is £528 for a householder application and £264 for a Lawful Development Certificate (England, from 1 April 2025; verify before applying).
Our own fee is fixed once quoted. The quote tool gives an indicative figure in minutes and a written quote follows within 24 hours.
Primary sources
- The Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2: the permitted development limits quoted above.
- Planning Portal: do you need permission: the national guidance and the application service every council accepts.
- London Borough of Barnet: planning: the council's own constraints map, validation checklist and pre-application service.
