House extension plans in Harrow, £895 fixed

What Harrow Council asks of a house extension

From the council's Residential Design Guide SPD (adopted 15 December 2010, council page).

• Where planning permission is required for a single storey rear extension, the acceptable depth will be determined by the need for consistency with permitted development (described in the guide as 4 metres on a detached house and 3 metres on a semi-detached or terraced house, from the rear wall of the original house), together with site considerations, scale, neighbour amenity and the character of the area. (source, checked 15 October 2026)
• Where a greater rear extension depth is acceptable, the additional depth should be set away from any side boundaries of the property by twice the amount of additional depth sought (the "two for one" rule). Chamfering of extensions is not normally acceptable, except for a purpose-built conservatory. (source, checked 15 October 2026)
• Subject to site considerations, the finished height of a single storey rear extension abutting a residential property should be a maximum of 3 metres on the boundary for a flat roof, and 3.5 metres at the mid-point of the pitch at the site boundary for a pitched roof. (source, checked 15 October 2026)
• In general there is no potential for first floor or two storey rear extensions on mid-terraced properties; where such extensions abut a side boundary and need planning permission they must always comply with the 45 Degree Code, and siting south or west of the neighbour would normally be unacceptable. (source, checked 15 October 2026)
• Conservatories sited within 3 metres of a boundary would normally be required to have brick flank walls or be finished with solid panels. (source, checked 15 October 2026)

What Harrow house extension drawings have to show

Harrow's guide measures from the rear wall of the original house, so the drawings dimension the depth from that wall. The 3 metre and 4 metre figures are how the 2010 guide describes permitted development, used as a consistency benchmark; they are not a statement of the current permitted development limit, which comes from the General Permitted Development Order. The guide also says site considerations, scale, neighbour amenity and the character of the area count. Where greater depth is acceptable, the additional depth is set away from any side boundary by twice the additional depth sought (the "two for one" rule), so 1 metre of extra depth would sit 2 metres from the side boundary. Chamfering is not normally acceptable, except for a purpose-built conservatory.

Height is drawn on the boundary elevation. Subject to site considerations, the guide says a single storey rear extension abutting a residential property should be a maximum of 3 metres on the boundary for a flat roof, and 3.5 metres at the mid-point of the pitch at the site boundary for a pitched roof. In general there is no potential for first floor or two storey rear extensions on mid-terraced properties; where one abuts a side boundary and needs planning permission it must always comply with the 45 Degree Code, and siting south or west of the neighbour would normally be unacceptable. Conservatories within 3 metres of a boundary would normally need brick flank walls or solid panels.

The council's requirements (checked 10 October 2026) list existing and proposed floor plans and elevations, site sections, finished floor and site levels, and roof plans, with the CIL form, and a detailed materials schedule in conservation areas or next to listed buildings. Missing information must be supplied within 21 days or the application is disposed of. A draft Small Sites Design Code, consulted on in 2021-22, proposed replacing the guide's householder section; the council's residential design page still links the 2010 guide (15 October 2026).

What it costs in Harrow

• Planning drawings: £895, plus £548 to Harrow Council.
• Building regulations drawings: £795.

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