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 Brentwood
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 Brentwood are listed below, taken from our Brentwood area page. Each one is a reason to confirm the address on Brentwood's constraints map before relying on permitted development.
- Green Belt. Most of the Brentwood district outside the built-up areas of Brentwood, Shenfield, Hutton and Ingatestone is Metropolitan Green Belt, including the villages of Doddinghurst, Kelvedon Hatch, Blackmore and West Horndon. Extensions to houses in the Green Belt must not be disproportionate to the original dwelling, and outbuildings face a stricter test on size and siting. Check the council's constraints map for whether your plot is inside or outside the boundary. Source
- Conservation areas. Brentwood has conservation areas covering the town centre around the High Street, parts of Shenfield, Ingatestone, Hutton and Warley, and several village centres. Roof extensions are not permitted development inside them, and officers apply the character appraisal to extensions and dormers. Check the council's constraints map for the exact boundaries before relying on permitted development. Source
- Thorndon Country Park and ancient woodland. Thorndon Country Park south of Brentwood, with Weald Country Park to the west, is protected open space with areas of ancient woodland and nature conservation designations. Houses backing onto the parks or onto protected woodland can face additional requirements on siting, trees and lighting; raise it with the council early. Source
- River Wid and Ingrebourne flood risk. Land alongside the River Wid around Ingatestone and Mountnessing, and along the upper Ingrebourne and its tributaries towards Warley and Pilgrims Hatch, sits in Flood Zones 2 and 3. Ground-floor extensions and basements there may need a flood risk statement with the application. Check the government flood map for your address. Source
- Listed buildings. Brentwood High Street, Ingatestone, Shenfield and the villages contain listed houses, inns and churches, with Ingatestone Hall and the chapel ruins in the town centre among the best known. Listed building consent is needed for almost any alteration, internal or external. Source
What Brentwood planning officers look for
Brentwood officers assess householder applications against the Brentwood Local Plan 2016–2033 and national Green Belt policy. On the inter-war semis and detached houses that dominate Shenfield, Hutton and Pilgrims Hatch they tend to look for two-storey side extensions set back from the front wall and below the main ridge, a gap kept to the side boundary so pairs of semis do not read as a terrace, rear extensions that clear the 45-degree line to the neighbour's nearest habitable window, and roofs that match the existing pitch and materials. In the Green Belt the first question is whether the extension, added to anything built since the house was original, is disproportionate; officers usually measure this by floorspace and volume, and proposals over roughly a third to a half of the original tend to need a strong case. Common refusal reasons are disproportionate Green Belt additions, terracing effects from side extensions, full-width box dormers and front dormers in conservation areas. Outside conservation areas and the Green Belt, hip-to-gable and rear-dormer lofts within 50 m³ on a semi are routinely lawful and we secure a Lawful Development Certificate as proof. Brentwood offers a paid pre-application service. Decisions can run past the statutory 8 weeks; allow 8–12 weeks.
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.
