# House extensions in Harrow

Canonical URL: https://mirion.uk/areas/harrow/extensions/

> Rear, side and two-storey house extensions in Harrow, with the council's Residential Design Guide rules on depth, height, set-backs and the 45-degree code.

We build single-storey rear, side and two-storey extensions across Harrow, designed around the council's Residential Design Guide and its 45-degree code from the start.

### Single-storey rear extensions

Harrow's guide links its expectations to permitted development: 3 metres on a semi-detached or terraced house and 4 metres on a detached house. Greater depth may be accepted where the extension is away from the boundary, the rear building line is staggered or the neighbour has already extended, but the extra depth should be set in from the side boundary by twice the additional amount \(the “two for one” rule\). Heights should be kept to 3 metres on the boundary for a flat roof, or 3.5 metres at the mid-point of a pitched roof.

### Side and two-storey extensions

Single-storey side extensions are normally acceptable up to a side boundary, subject to neighbours' windows and the street scene. First-floor and two-storey side extensions to detached and semi-detached houses should be set back at least 1 metre from the front wall with a subordinate pitched roof, preferably hipped, to avoid a terracing effect. The guide says there is generally no scope for first-floor or two-storey rear extensions on mid-terraced houses.

### The 45-degree code

Two-storey elements are tested against a 45-degree line from neighbours' protected windows, so the position of their windows often decides the size of a first-floor extension. We measure these on site before drawing anything, and check Harrow's 30 conservation areas and Article 4 directions for your address.

Harrow extensions: common questions

How far can I extend at the back in Harrow?

Without planning permission, 3 metres on a semi-detached or terraced house and 4 metres on a detached house, or up to 6 and 8 metres through prior approval. With an application, Harrow uses the same depths as a starting point and may accept more where the site allows, applying its “two for one” rule.

Can I build a two-storey rear extension on a terraced house in Harrow?

Rarely. Harrow's Residential Design Guide says there is generally no potential for first-floor or two-storey rear extensions on mid-terraced houses because of the effect on neighbours. End-of-terrace houses are assessed like semi-detached houses.