Side extensions 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 side extensions
A single-storey side extension is permitted development on a house if it is no wider than half the width of the original house, no higher than 4 m, with eaves no higher than 3 m where it comes within 2 m of a boundary, and finished in materials similar in appearance to the house.
Anything wider than half the original house, any two-storey side extension, and any side extension on a flat needs planning permission. Garage conversions that keep the walls are usually permitted development; extending beyond them is assessed as a side extension.
What changes in Barnet
Side extensions are not permitted development in a conservation area at all, so there a planning application is always required.
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
Barnet officers apply the Residential Design Guidance SPD closely. 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.
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.
