A side-return extension infills the narrow strip of ground that runs alongside a rear closet wing — the two-storey back projection that typically carried the original scullery below and a bathroom above. That form belongs to a particular kind of house, and in Golders Green it is the minority rather than the rule. The district grew rapidly after the Northern line arrived in 1907, and its dominant stock is 1920s–30s semi-detached and detached houses, larger double-fronted villas and purpose-built mansion flats. The genuine side-return candidates are the Edwardian and 1900s bay-fronted houses and the interwar terraced houses, where a closet wing and a boundary leave a usable alley between them. Everything else on this page follows from that one survey question, because whether the alley exists decides the structure, the drainage problem, the roof geometry and the planning argument that come after it. It is worth answering honestly at the outset rather than discovering three months in that the project you costed is not the project your house supports.
On a semi-detached or detached house the space beside the house is not a return but a flank: a full-height side elevation facing a driveway, a path or the neighbouring pair. Building into it is a side extension, and it behaves differently in almost every respect. It is seen from the street rather than hidden behind the house, so it is judged on the gap between buildings and the rhythm of the pair as much as on rear amenity; it generally wants to be set back from the front elevation and kept subordinate to it; and it usually reaches up rather than simply out. Calling a flank extension a side return is an easy briefing error to make on this stock, and it sends the design, the cost expectation and the consent route in the wrong direction from the first sketch — which is why we settle the vocabulary at feasibility, before a line is drawn and before a consent route is chosen.
Infilling a true return means two structural moves, not one. The closet wing's flank wall — often a single-skin wall on shallow original footings — either becomes an internal wall or is taken out entirely, and where it comes out it is carrying the wing's roof and the floor above it, so a beam and properly sized padstones have to take that load. Separately, opening the original rear wall of the main house to win a full-width room is a second beam on a second line, and the two frequently meet at an internal corner where their bearings compete for the same pier. The foundations beneath both walls are rarely deep enough by modern standards, so a new foundation going deeper alongside them can require underpinning, and the sequence in which the two walls are opened matters as much as the sizes on the calculation sheet. We bring a structural engineer in at feasibility rather than after planning, because the beam layout and the underpinning strategy both change where the party wall notices land — and a notice served on the wrong basis has to be served again.
Where a true return exists, the alley you want to build on is usually the house's services corridor as well, and that is the constraint most likely to reshape the design once the ground has been surveyed. The closet wing's rainwater downpipe usually discharges into it, the soil stack from the first-floor bathroom commonly runs down the wing's flank, and on houses of this age an inspection chamber frequently sits inside the return itself. All three occupy precisely the ground the new foundation and floor slab want. A stack can often be re-routed internally or absorbed into the new envelope, but a shared or public run passing under the footprint needs a build-over agreement with the water authority, and a chamber has to be relocated or kept accessible in its new position. We survey the drainage and lift every cover before the layout is fixed — finding a shared run beneath a poured slab is an expensive way to learn what was under the alley.
The roof over the infill is where the design is usually decided, and the governing constraint is the closet wing's own first-floor windows. They look into the alley — a bathroom or a small bedroom, characteristically — and the new roof has to pass beneath them. Where the roof would cut across a window, that window must be dealt with honestly rather than drawn out of the elevation: raised, replaced with a high-level opening, or accepted as lost with the room served another way, which building regulations will test for light and ventilation. That single decision fixes the roof height, and the roof height in turn fixes whether the infill is a flat roof with rooflights or a shallow mono-pitch falling away from the wing. We set that section early, because it also governs how much daylight the new room can actually take and how the addition is read from a neighbour's upper windows. A return infill that sits low against the wing is usually a far easier neighbour case than a deep rear projection of the same floor area.
Both edges of the alley are boundaries in law even when only one of them is a wall. On a terraced house the infill is built on or up to the line of junction, and where the neighbouring house is a mirror image its own alley may run alongside yours, so the low wall between the two returns is the very thing being built on. Building on the line of junction, or excavating within three or six metres of a neighbour's structure, brings the Party Wall etc. Act 1996 into play — a process that runs entirely separately from the planning decision and cannot be cured by it. Right to light likewise remains a private civil matter that consent does not settle. We identify the notifiable neighbours at feasibility, confirm the designation position with Barnet, prepare the drawings the planning and building-regulations routes each require, and are candid that no approval can be promised.
