The right conversion depends on the roof you start with. Wandsworth's bay-fronted Victorian terraces typically carry a pitched, slated front slope that the street reads as a continuous set, so the floor area almost always comes from the rear slope — a box dormer that lifts headroom across the back of the plan while the front pitch stays untouched. Edwardian terraces often have a steeper roof and more ridge height to work with, and an end-of-terrace or semi-detached house adds the option of a hip-to-gable widening that squares off the hipped end before the dormer is formed. Rooflight conversions keep the roof shape entirely and raise the fewest planning questions, but add the least usable height, so they suit an already tall roof rather than a marginal one. We match the option to your roof and to what the street can carry, not to a template, and we say early when a roof simply will not give a full room.
Head height under the ridge decides whether the loft is worth doing at all. As a rule of thumb you want roughly 2.2 to 2.4 metres from ceiling joist to ridge before the new floor and ceiling build-up eat into it, and the finished room has to hold a usable height across enough of its footprint rather than only along the centre line. Many of the taller terraces near the commons convert cleanly; a shallower or lower-ridged roof may only yield a full-height room once a dormer is added to the rear slope, which is one reason the dormer question and the head-height question are answered together at survey rather than in sequence. A tape measure to the ridge on a first visit tells us quickly whether the roof is a straightforward convert, a dormer-dependent one, or a case where the gain is too marginal to justify the disruption below.
The new staircase is the part owners most often underestimate. It has to rise from the storey below within the going and rise limits of Approved Document K and land with full headroom over it, and on a two-storey terrace it almost always takes a slice out of a rear bedroom or the landing beneath. On the narrow terrace plan typical here, positioning the flight over the existing stair, so the runs stack, is usually the move that keeps a bedroom intact — but the outrigger, the party walls and the ridge line all constrain where it can go, and headroom at the top of the flight often forces the dormer or a rooflight to sit directly above the landing. We fix the stair position at feasibility because it governs both the cost to the floor below and whether the roof space is genuinely worth converting, and because a stair resolved late almost always erodes the very bedroom the loft was meant to add.
Fire escape is the requirement that catches people out. Turning a two-storey terrace into three storeys lifts the new floor above the 4.5-metre threshold and triggers a higher standard: a protected stair enclosure running all the way down to a final exit, fire-rated doors — typically FD30 — to the habitable rooms off it, and mains-powered, interlinked smoke alarms. On an open-plan Victorian ground floor that has already lost its hallway to a through-lounge, enclosing the stair cleanly can be the hardest part of the whole job, so an alternative such as a sprinkler or mist system, or a compliant escape window served by the layout below, is sometimes designed in. We build the escape strategy into the drawings from the start rather than letting building control reshape the layout late, because a fire line drawn after the plan is fixed tends to reintroduce the very partitions the owner opened up in the first place.
A converted roof needs new structure, and on a terrace that structure lands on shared walls. Typically steel beams span the house at the new floor level and at the ridge, bearing onto or into the party walls on either flank, with the existing rafters, purlins and any binders reworked to a structural engineer's calculations. Because those bearings sit in the party wall, the Party Wall etc. Act 1996 is engaged and notices go to the neighbours on both sides before work starts — a mid-terrace loft usually notifies two adjoining owners, not one. The roof is also where the insulation is brought up to current standards, whether the slope is kept or opened up for the dormer, and where the existing rafter depth rarely leaves room for enough insulation without either dropping the ceiling or building out — another trade-off against the head height that has to be settled at design stage rather than on site.
For a mid-terrace loft the dormer detailing is where a scheme is won or lost. Set the box dormer back from the eaves and down from the ridge and clad its cheeks in a material that recedes — often slate-grey or a dark standing-seam — and it reads as a quiet addition to the rear roofscape; run it full-width, hard to the eaves and in a pale render, and it looks bolted on and invites objection, particularly where a conservation area applies. Rear-facing glazing, the flat-roof or shallow-pitch top and the junction with the neighbour's roof at the party wall all get detailed so the addition sits below the ridge line and keeps the terrace's rhythm legible from the gardens behind. Where neighbours on the same run have already converted, matching the established set-backs and materials of those dormers is often the quickest way to demonstrate the scheme is in keeping and to head off amenity objections about overlooking or bulk.
