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. Several have Article 4 directions removing permitted development for front alterations, front boundary walls and sometimes roof works. Check the council's conservation area pages and constraints map for your street. 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. Ground-floor extensions here may need a flood risk statement. 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. Plots backing onto them face closer scrutiny for outbuildings and large extensions. 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. We are not certain of the current extent, so confirm with the council before relying on prior approval. Source
What Brent planning officers look for
Brent officers lean heavily on the Design Guide SPD. 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
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.
