Does my existing extension use up the garden room allowance in Harrow?

It can. Planning Portal's permitted development condition is that no more than half the land around the original house is covered by additions and other buildings, and Harrow's Residential Design Guide (paragraph 6.19) says extensions and outbuildings above 50 per cent of the rear garden are unlikely to be permitted. The rear extension and any shed or garage count in the total, so we measure the garden before sizing the room.

Can I build a garden room if I live in a flat in Harrow?

Not under permitted development. Paragraph 6.77 of Harrow's Residential Design Guide says that if you live in a flat you need planning permission for any outbuilding or shed. The application is then judged on siting, height and neighbour impact, using the paragraph 6.78 guidance as a yardstick.

Does a garden room in a Harrow conservation area need planning permission?

Not always, but it depends on the address. Planning Portal says outbuildings at the side of a house on designated land, which includes conservation areas, need permission, and an Article 4 Direction can remove further rights. Harrow lists directions for parts of Stanmore, Harrow on the Hill, Harrow Weald and Pinner, so we check the direction for your street and ask the Conservation Team before relying on permitted development.

Does a garden office need Building Regulations approval?

Not if it is under 15 m², or under 30 m² with no sleeping accommodation and either built substantially of non-combustible materials or set at least 1 m from every boundary. Electrics still need a certificate from a registered electrician.

Can I put a bathroom in it?

A WC or shower does not by itself take an outbuilding out of permitted development, as long as the building remains incidental to the house and is not used for sleeping. Drainage may need building control notification.

Why does 2.5 m matter so much?

Most London gardens are narrow, so the outbuilding ends up within 2 m of a boundary, where the overall height limit is 2.5 m from the highest adjacent ground. That governs the roof build-up and floor level, which we design to the millimetre.

How long does Harrow take to decide?

The statutory period is 8 weeks from validation for both a householder application and a Lawful Development Certificate, and 42 days for a larger home extension prior approval. The council offers a paid pre-application service and decisions often run beyond 8 weeks, so allow 10–14 weeks.

Garden Rooms and Outbuildings in Harrow

A garden room in Harrow usually starts with a 1930s semi. In Rayners Lane, Kenton, North Harrow and Belmont the plot is long and narrow, so the building ends up near a side boundary and its height and position decide the route. The detached inter-war houses of Stanmore, Hatch End and Pinner have wider gardens but more often sit in a conservation area or beside Green Belt land around Harrow Weald Common. In Wealdstone and Roxeth the Victorian terrace garden is shorter, and a flat has no garden building rights at all, so we check the house type and the council's constraint maps before drawing.

The permitted development rules for garden rooms and outbuildings

An outbuilding is permitted development on a house if it is single storey, no higher than 2.5 m where it comes within 2 m of a boundary, otherwise no higher than 4 m with a dual-pitched roof or 3 m with any other roof, sits behind the front wall of the house, covers together with other outbuildings no more than half the land around the original house, and is used for purposes incidental to the house rather than as a bedroom or self-contained flat.

Anything used for sleeping or as a separate dwelling needs planning permission regardless of size. Flats have no permitted development rights for outbuildings, and listed buildings need listed building consent.

What changes in Harrow

In a conservation area an outbuilding is still permitted development behind the house, but not to the side of it, and the same height limits apply.

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's Residential Design Guide asks of a garden building

Harrow's Residential Design Guide SPD (2010), which the council still lists on its residential design page, has a short section on outbuildings. Paragraph 6.78 says a structure should be sited in the final quarter of the garden and be proportionate to the size of the original dwelling. It should normally stand at least two metres from the boundaries, in which case it should not exceed 4 metres with a dual-pitched roof or 3 metres with any other roof. Within 2 metres of a boundary it recommends a maximum of 2.5 metres, and says that unless the building is finished in timber, its external finish should usually match the house.

Paragraph 6.77 says that anyone living in a flat needs planning permission for any outbuilding or shed. Paragraph 6.19 adds that extensions, together with existing or proposed outbuildings and other structures, that take up more than 50 per cent of the rear or other main garden are unlikely to be permitted, because they harm the character of the area. On a semi that already has a rear extension and a shed, we add up the garden coverage before choosing the size of the room.

Green Belt, Metropolitan Open Land and conservation areas

Harrow's Development Management Policies Local Plan (2013) treats Metropolitan Open Land with equal weight to the Green Belt (paragraph 5.7). Paragraph 5.8 says residential gardens are not previously developed land, and that extensions and alterations to dwellings inside the Green Belt are assessed against national policy and the London Plan. Policy DM 16 (D) says proposals that are inappropriate or harm the openness of either designation will be refused without very special circumstances. Paragraph 5.10 names the slopes of Harrow on the Hill and Stanmore Golf Course as Metropolitan Open Land, so a garden room near either is worth checking against the policies map before design starts.

In conservation areas, Planning Portal says that on designated land an outbuilding at the side of the house needs planning permission, and the same applies to any outbuilding within the curtilage of a listed building. Harrow's conservation pages show Article 4 Directions in parts of Stanmore and Edgware, Harrow on the Hill, Harrow Weald and Pinner, but do not say which rights each removes. The Stanmore and Edgware directions proposed in a March 2020 planning committee report covered front alterations, boundaries, roof works and accesses, not garden buildings. Other areas may differ, so confirm the position for your own address with the Conservation Team.

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

Harrow Council: Residential Design Guide SPD (2010)
Harrow Council: Development Management Policies Local Plan (2013)
Harrow Council: Stanmore and Edgware conservation areas
Harrow Council: Proposed Article 4 Directions in Stanmore, planning committee report (11 March 2020)
Harrow Council: Pinner conservation areas
Planning Portal: Outbuildings, planning permission
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.