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. The Hampstead Garden Suburb Trust also has to approve alterations under the scheme of management. Budget extra time. Source
- Conservation areas. Barnet has numerous conservation areas, including Hampstead Garden Suburb, Monken Hadley, Totteridge, Mill Hill, Finchley Church End and Hendon (The Burroughs). Roof extensions are not permitted development in these areas, and the council expects extensions to be subordinate and in matching materials. Source
- Green Belt. Large tracts of Totteridge, Arkley, Mill Hill and the northern fringe of the borough are Green Belt. Extensions to existing houses are allowed in principle but must not be disproportionate to the original dwelling; outbuildings face a similar test. Check the Local Plan policies map for your plot. 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. We are not certain of the full list, so check the council's constraints map before building under permitted development. 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. Decision times often exceed the statutory 8 weeks; a written extension of time is common.
Fees and timelines
The council fee for a householder application is £528 and for a Lawful Development Certificate £264 (England, from 1 April 2025; verify before applying). Both are decided within 8 weeks of validation.
Our own fee for the drawings and the application is fixed once quoted and depends on the project type, its size and the constraints above. Answer six questions in the quote tool and we email you an indicative figure within minutes, then a written fixed-fee quote within 24 hours.
