Side extensions in Harrow depend on the house. Typical homes here: 1930s semi-detached houses in Rayners Lane, North Harrow, Kenton, Belmont, Harrow Weald and Pinner; Detached inter-war houses in Stanmore, Hatch End, Pinner and around Harrow Weald Common. Designations to check first: Green Belt and Metropolitan Open Land; Conservation areas; Article 4 directions. Applications are decided by London Borough of Harrow and the Harrow Local Plan (Core Strategy 2012 and Development Management Policies 2013).
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 Harrow
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 Harrow are listed below, taken from our Harrow area page. Each one is a reason to confirm the address on Harrow's constraints map before relying on permitted development.
- Green Belt and Metropolitan Open Land. The north of the borough is Green Belt, including Harrow Weald Common, Stanmore Common, Bentley Priory, Pinner Park Farm and Old Redding, with Metropolitan Open Land at Harrow on the Hill and along the Yeading Brook. Source
- Conservation areas. Harrow has around 28 conservation areas, including Harrow on the Hill, Pinner High Street, Pinnerwood Park Estate, Roxeth Hill, Harrow Weald Park, Stanmore Hill and Kerry Avenue. Source
- Article 4 directions. Pinnerwood Park Estate and several other conservation areas have Article 4 directions removing permitted development for front alterations, windows, boundary treatments and some roof works. Source
- Listed buildings. Harrow on the Hill, Pinner High Street, Headstone Manor and Bentley Priory contain concentrations of listed buildings, including several medieval and Tudor houses in Pinner. Source
- Local flood risk. The River Pinn, Yeading Brook and Wealdstone Brook put parts of Pinner, Eastcote Lane, Wealdstone, Kenton and Headstone in Flood Zones 2 and 3, with surface water flooding elsewhere. Source
What Harrow planning officers look for
Harrow officers are, in our experience, reasonable on suburban householder work but apply the Residential Design Guide closely. Common refusal reasons are two-storey side extensions without a set-back from the front wall that create a terracing effect, rear extensions that fail the 45-degree test, front dormers, and box dormers that wrap the hip. The council offers a paid pre-application service and decisions often run beyond 8 weeks, so allow 10–14 weeks.
Fees and timelines
Harrow 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 Harrow: planning: the council's own constraints map, validation checklist and pre-application service.
