A side return in Belgravia rarely resembles the suburban alley the phrase suggests. On a stucco terrace the back of the house steps out into a closet wing, and the strip left beside it is a narrow, deep slot, commonly walled on both sides by tall neighbouring brickwork. Because so many of these houses drop to a garden or lower-ground floor, that slot commonly sits a storey beneath the raised ground level, half yard and half lightwell, with only a shallow garden beyond it. Roofing it over will not deliver the sprawl of floor area the same move buys in an outer London terrace, and anyone hoping for that is measuring the wrong thing. What an infill can deliver is width: a garden-level room that finally runs the whole way across the plan, properly connected to the garden in a way the original service arrangement never allowed, and a kitchen that is no longer a corridor with appliances along one side.
The first question is not how much floor you gain but what you take away. In a house of this type the slot is frequently the only source of daylight and natural ventilation reaching the rooms behind it at garden level, and sometimes to a neighbour's wing windows facing across it. Roof the whole strip without thinking and you buy width at the price of a dark, airless core. So we plan the infill as a light strategy rather than a footprint: a continuous run of rooflights along the line the slot used to occupy, a retained shaft or set-back where an existing window has to keep what it has, and glazing at the garden end proportioned to the depth of the new room rather than to the size of the opening. Where a neighbour is affected, daylight and sunlight are tested to the BRE method, and right to light stays a separate civil matter that a planning consent does not settle.
The second question is whose ground you are building on, and it is answered by documents as much as by drawings. The slot is bounded by a flank that may be a genuine party wall, the neighbour's independent wall, or a boundary shared only in part, and in a terrace divided into flats the rear strip and the garden may be demised to the lower unit rather than to the house as a whole. None of that is safe to take on trust. It needs a measured survey, the title plan and, where a lease exists, the demise read alongside them. What the historic footings under these walls can actually carry is never safe to assume, so an engineer has to open up and establish what may be enclosed upon, what needs underpinning and what must simply be built independently alongside, before a single line of the new plan is committed.
Then there is the fabric you have to cut. Making the room work means forming a wide structural opening between the wing and the main rear room, and usually a second opening through the rear wall to the garden. Where the house is listed, and a great many here are, that is protected fabric: the wing, the rear elevation, the chimney breast and the original plan form all fall within what listing controls, so consent is a separate application and the design has to be argued, not simply drawn. The roof of the infill deserves the same care, because it is typically looked down on from the wing's own windows, from the upper floors and from neighbouring gardens. Treated as a fifth elevation, in lead, zinc or a planted finish with disciplined rooflights, it belongs to the house; treated as leftover, it advertises the extension from every window above.
The slot is also where the terrace tends to hide its plumbing. The closet wing was typically built as a service wing, so the soil stack that served it, the downpipes shedding the roof above, a gully and often an inspection chamber can all land in precisely the strip you want to build on, and at garden level they may run beneath the level of the new floor. Each one needs a survey and a designed answer: a stack rerouted and refixed, which on a listed rear elevation is itself consentable work; a chamber raised, moved clear of the footprint or kept reachable under a sealed, screwed-down cover; and, where the run is a public sewer, a build-over agreement with the water authority. Finding a shared drain after the steelwork has been designed is among the most reliable ways to lose both money and programme.
Finally, the route. Householder applications are determined in around eight weeks, but that clock is only one of several running at once. Where the property lies within a conservation area, permitted development is curtailed and the position must be confirmed for the exact address rather than borrowed from a neighbour's earlier scheme. Where the house is listed, consent is a separate application on separate grounds. Where the freehold or a long lease sits with the Grosvenor estate, a Licence to Alter and the estate's own review of method and materials run on their own timescale, and a managing agent may want the structural approach seen before anything is served on a neighbour. We map those tracks at the outset and run them in parallel, and we are straightforward that we do not promise approval on any of them.
