Side extensions in Brent depend on the house. Typical homes here: Edwardian terraces and semis in Willesden Green, Kensal Rise, Mapesbury and Queen's Park; Victorian terraces and mansion blocks in Kilburn and Harlesden. Designations to check first: Conservation areas and Article 4 directions; Flood risk: River Brent and Wealdstone Brook; Metropolitan Open Land and Fryent Country Park. Applications are decided by London Borough of Brent and the Brent Local Plan 2019–2041.
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 Brent
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 Brent are listed below, taken from our Brent area page. Each one is a reason to confirm the address on Brent's constraints map before relying on permitted development.
- Conservation areas and Article 4 directions. Brent has numerous conservation areas, including Mapesbury, Queen's Park, Kensal Rise, Willesden Green, Mount Stewart and Sudbury Court. Source
- Flood risk: River Brent and Wealdstone Brook. Parts of Alperton, Stonebridge, Wembley and Neasden along the River Brent, the Grand Union Canal and the Wealdstone Brook sit in Flood Zones 2 and 3 or carry surface-water risk. Source
- Metropolitan Open Land and Fryent Country Park. Fryent Country Park, the Welsh Harp (Brent Reservoir) edges and Barham Park are Metropolitan Open Land or protected open space. Source
- HMO and Class MA Article 4 directions. Brent may have Article 4 directions restricting changes to houses in multiple occupation and, in some town centres, commercial-to-residential conversions under Class MA. Source
What Brent planning officers look for
Common refusal reasons are rear extensions deeper than the neighbour's that fail the 45-degree test, two-storey side extensions without a set-back that create a terracing effect, and roof extensions in conservation areas or oversized dormers that wrap the hip. Rear dormers outside conservation areas are generally accepted if they sit below the ridge and inside the roof slope. Brent offers a paid pre-application service and decision times often run past the statutory 8 weeks, so allow 10–14 weeks.
Fees and timelines
Brent 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 Brent: planning: the council's own constraints map, validation checklist and pre-application service.
