Two-storey 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 two-storey extensions
A two-storey rear extension is permitted development only within tight limits: no more than 3 m deep, at least 7 m from the rear boundary, no higher than the existing house, eaves matching the existing eaves, materials similar in appearance, and any upper-floor side window obscure-glazed and non-opening below 1.7 m.
Most two-storey extensions exceed one of those limits, most often the 7 m rear boundary rule on London plots, so the usual route is a householder planning application. Flats have no permitted development rights.
What changes in Harrow
Extensions of more than one storey are not permitted development in a conservation area, so planning permission is always needed there.
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
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.
