A rear extension in Wandsworth starts with the existing rear building line — the back wall of the main house and, on the typical terrace, the return wall of the two-storey back addition beside it. The useful question is how far a new single-storey addition can project past that line before it erodes the garden, overshadows the neighbours either side or reads as an over-development of a plot that has to match its terraced neighbours. We survey the back of the house, plot the boundary positions and set a projection that is both liveable and defensible, rather than reaching for the deepest footprint the garden will physically take. On a mid-terrace house that line is fixed by the two adjoining houses; on an end-of-terrace or semi it can also step around the flank, but the rear projection is still the primary move and the one the case officer weighs first.
The full-width rear extension is the move this stock is built for. A classic Wandsworth terrace has a two-storey back addition set to one side, leaving a shallow original garden step beside it; squaring the ground floor off to the full width of the plot captures that leftover space and gives a single deep room across the back of the house. The design turns on how the new flat or lightly pitched roof meets the retained back-addition wall above it, where the rainwater goes now that the roof spans wall to wall, and how the extra width sits against both side boundaries at once — a different problem from infilling a single side passage. It also changes the structure, because the opening back into the original house widens from a single doorway to a full-width span that needs a beam sized for the load above and bearings the party walls can carry.
Depth and daylight are decided together, in section rather than on plan. A rear extension that runs the full width of a terrace throws a longer shadow across the gardens on either side than a narrow one of the same depth, so the two neighbours' ground-floor windows and their garden light set the ceiling on how far the build can go. We model the projection against a notional 45-degree line from the neighbours' nearest windows and against the boundary heights, then fix a depth that keeps the scheme inside the borough's amenity expectations before any drawing is committed to a planning submission. Because the assessment is two-sided on a terrace, the constraining neighbour is often the one whose rear windows sit closest to the shared boundary, and we test against that worst case rather than an average.
The single-to-two-storey decision changes the assessment entirely. A single-storey rear extension is judged mainly on garden loss and neighbour daylight; adding a first storey over it brings outlook, a sense of enclosure and overshadowing from a raised rear wall into play, and on a terrace where the houses share both flanks that is far more sensitive. On the deeper family-house plots near the commons a two-storey rear addition can be realistic, but we advise early which storey height a given house and plot can carry, because designing a two-storey scheme that only a single storey can defend wastes the design fee. A part-two-storey solution — the upper floor set in from one boundary, or stepped down in height near it — is often what makes the extra bedroom achievable where a full-width two-storey wall would not be.
Roof form and level are where a full-width rear extension is resolved technically. A flat roof with a discreet upstand, a shallow mono-pitch falling away from the house or a set-back roof behind a low parapet each suit a different rear elevation and a different relationship to the retained back-addition windows above. Getting the extension roof to sit below those first-floor windows so they keep their light, detailing the junction where the new roof meets the old return wall so it stays weathertight, and running the drainage off a wide roof to a sensible discharge point are the details that make the difference between a clean build and a leaking parapet. Where the plan calls for rooflights to bring daylight deep into a room that now sits well back from the garden wall, their position is set against the structure spanning below rather than dropped in afterwards.
From there the route is a coordinated one. We survey the house, test the extension against the planning, daylight and boundary constraints, and develop a rear addition that a Wandsworth case officer can support, then produce the planning drawings and, once the principle is settled, the building regulations package that covers the new structure, foundations, insulation, drainage and weathering for construction. On terraced plots the Party Wall etc. Act 1996 almost always applies to a rear extension, so we coordinate the notices with a party wall surveyor, and where the new foundations sit over or near a shared drain we design the build-over so building control and the water authority are satisfied. The result is a rear extension that adds a genuinely usable room across the back of the house without unbalancing the garden or the neighbours.
