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. Extensions to houses in the Green Belt must not be disproportionate to the original dwelling, and outbuildings face a strict test. 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. Roof extensions are not permitted development inside them and extensions are expected to be subordinate and in matching materials. 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. The council has also considered a direction on small HMOs; check the constraints map and current position before relying on permitted development. 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. Listed building consent is needed for almost any alteration. 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. Ground-floor extensions there may need a flood risk statement. 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. Hip-to-gable lofts with rear dormers and 4–6 m rear extensions on ordinary 1930s semis are usually accepted. The council offers a paid pre-application service and decisions often run beyond 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.
