A rear extension on a Clapham Junction terrace starts with reading the existing rear building line. These Victorian and Edwardian houses typically have an original back addition — the closet wing — projecting into the garden alongside a narrow side return, which leaves the ground floor an awkward L-shape. A full-width single-storey extension squares that plan off, bringing the rear wall out level across the whole width of the house. The useful question is not how big the addition could be, but how far past the original rear wall it can project before it erodes the garden, overshadows a neighbour or reads as an over-development of a tight terraced plot.
Neighbour daylight is the constraint that most often sets the depth. Because a terrace sits wall-to-wall, a full-width rear extension built to both flank boundaries can affect the ground-floor windows of the houses on either side, and Wandsworth assesses that impact using the established BRE daylight and sunlight tests. A projection that is unremarkable on the deeper Wandsworth Common edge plots can be resisted where the terrace and its gardens are shorter. We model the extension in section against each neighbour's rear window early, so the depth is agreed on evidence rather than optimism and the scheme is defensible when it reaches a case officer.
Plot depth changes the answer street by street. The terraces on the Wandsworth Common edge often have longer back gardens that can absorb a deeper rear extension while still leaving usable amenity space, whereas the tighter plots nearer the station and around Northcote Road leave far less room to project before the remaining garden becomes a token strip. We measure the actual garden rather than working to a rule of thumb, because the same footprint that suits one house will over-develop another only a few streets away. The balance between new internal floor area and retained outdoor space is set from that survey.
The choice between a single and a two-storey rear extension changes the assessment entirely. A single-storey addition is judged largely on garden impact and neighbour daylight; a two-storey rear extension — realistic only where the plot depth and the party boundary allow — brings first-floor outlook, a sense of enclosure and potential overlooking of the neighbours' gardens into play, and on a mid-terrace it presses against two shared boundaries at once. We advise early which height a given house can carry, because committing to two storeys where the plot cannot support it wastes a planning submission that a single-storey scheme would have passed.
Roof form, materials and the junction with the old house are where a rear extension is won or lost. A flat roof with a discreet parapet, a shallow mono-pitch or a glazed lantern each suit different conditions, and the right choice depends on the first-floor window heights above and how the addition is seen from the garden. We detail the meeting of the new roof with the original single-skin back-addition wall so the extension reads as subordinate. Right to light is a separate civil matter that planning consent does not settle, so where the addition approaches a neighbour's window we flag it early rather than let it surface late.
From there the route is a coordinated one. We survey the house, test the rear extension against Wandsworth's amenity policies and the daylight constraints, and develop a design a case officer can support, then produce the planning drawings and, once the principle is settled, the building regulations package covering structure, drainage, insulation and fire. On a terrace the new opening in the rear wall usually needs a steel beam bearing into the party walls, so the Party Wall etc. Act 1996 applies to the neighbours on both sides. Identifying them at the outset keeps the build on programme rather than stalled at foundation stage.
