House
extensions
in Hammersmith, W6

The Town Hall profile points out that the building line at the rear, often a repeating pattern of subordinate original rear extensions, can matter in its relationship with the gardens, and asks that new rear extensions be designed with care to respect it.

The borough's guidance warns that the narrow gaps between back additions may already be barely acceptable for the rooms whose windows look into them, and that taking those additions further, above ground level especially, will generally be resisted. Key principle HS7 adds that an extension should not reduce the unobstructed view from an existing rear addition's window by more than 15%.

A new rear room is also a chance to deal with rain. The guidance says householder extensions can offer sustainable drainage through a water butt, permeable paving or a rain garden, and key principle SuD5 asks that flat roofs be living roofs, with at least 80mm of substrate, on developments of any size.

Listed houses need consent first: Hammersmith & Fulham says it is a criminal offence to extend or alter a listed building without listed building consent, even without knowing it was listed, and groups such as Angel Walk are listed Grade II.

Space behind the house can rule an extension out altogether: where amenity space is already limited or buildings stand close together, the borough's guidance says no rear extension at all may be appropriate. Key principle HS6 measures the effect on neighbours with a 45-degree line from the rear boundary, taken at ground level where the adjoining garden is less than 9 metres long and 2 metres up where it is longer.

What the street already has also counts. The justification to Policy DC4 says the council weighs the extensions a street already has when it judges whether a new one is appropriate, so we photograph the rear additions along the terrace before drawing ours.

Windows already looking onto the garden set a further test. Under key principle HS7 an extension at the back must leave the outlook no worse from any rear habitable room window below its level, and a ground-floor room in the back addition lit by a single window keeps an unobstructed 45-degree angle to it.

Outside Hammersmith: Belsize Park Gardens, NW3, a full-house refurbishment in Belsize Park that included an extension.

We draw the neighbouring back additions and their windows on the survey, so the outlook and light tests can be shown on the plans rather than argued after submission.

Sources: Hammersmith & Fulham: Hammersmith Town Hall Conservation Area character profile (November 2018); LBHF Planning Guidance Supplementary Planning Document (adopted February 2018); LBHF - Statutory listed buildings; LBHF Local Plan (adopted February 2018); Hammersmith & Fulham: Planning Guidance Supplementary Planning Document (February 2018).