On a typical Primrose Hill terrace the original back addition, the closet wing, runs down one side of the rear and leaves a narrow strip of open ground beside it: the side return. That strip is usually dead space, holding little more than drainpipes and a gully. Infilling it brings the outside wall out to the boundary and squares the plan, so a cramped galley kitchen becomes a full-width room. Because you are widening rather than projecting far into the garden, the massing is modest, but the technical detail across drainage, structure and the party wall is where these projects are won or lost.
The catch is light. Once you build across the return you lose the side window that lit the back of the plan, so the design has to put daylight back. On these terraces we usually do that from above, with a run of rooflights over the new infill, a glazed valley, or a lantern where the ceiling can take it, combined with large sliding or pivoting doors onto the garden. Getting the roof glazing right matters more here than in a deeper rear extension, because the infill is narrow and long and relies almost entirely on toplight rather than a wide garden wall.
The new flank wall sits on, or hard against, the boundary with your neighbour, which brings the Party Wall etc. Act 1996 into play. Building astride the line of junction, cutting into an existing party wall for the new roof, or excavating for foundations within three or six metres of the adjoining building all trigger notice. Primrose Hill's closely-set terraces mean the neighbour's own closet wing is often right there, so we design the foundations, the wall build-up and the weathering junction to work with their structure, and serve notice early to keep the programme on track.
Drainage is the other constant. The side return is where a Victorian terrace hides its rainwater downpipe, the kitchen waste, a back-inlet gully and often an inspection chamber. Infilling means rerouting those runs, keeping rodding access, and maintaining falls to the sewer under a new floor. If a public sewer crosses the return, Thames Water may require a build-over agreement, which is a separate consent to the planning application and worth establishing early. We survey the existing runs and set out relocated gullies and chambers as part of the drainage strategy, not as an afterthought on site.
To get the open kitchen-diner people want, the closet-wing flank wall usually has to come out and be replaced by a steel goalpost frame carrying the retained wing above. On older Primrose Hill houses that flank is frequently a single-skin, half-brick wall never meant to be structural, so it is rebuilt or underpinned as part of the works. We coordinate the structural engineer's beam and padstone design with the drainage and the party wall notices so the three do not collide, because the point where a steel bears onto the boundary wall is exactly where a party wall award, a manhole and a foundation can all want the same space.
Set within Camden's Primrose Hill Conservation Area, even a rear infill is judged on how it meets the conservation statement adopted in 2000. The good news is that a side return is generally the least visible extension a terrace can take, sitting behind the building line, below the ridge and screened by the wing itself. The care goes into matching brick and render, choosing rooflights that sit flush, and detailing the junction with the original wing so the rear elevation still reads as part of the terrace. We prepare the householder application and the technical package to carry that intent through to construction.
