The first question is not what you want in the roof but what roof you have and who can see it. A Victorian terrace on a Dartmouth Park slope does not present its roofs the way a terrace on level ground does: where the street falls, the ridge and eaves step from house to house, and a rear slope that would be genuinely private on flat land can be visible obliquely from further down the hill, from higher ground toward Highgate and the Heath, and from the gardens and upper windows behind. That changes the design brief. A rear dormer here has to be composed as something that will be looked at, not merely tolerated because nobody will notice it, and we survey the roof and its exposure before recommending a form.
Chimney stacks are the second controlling element, and they are usually the one owners have not planned for. On a terrace the stacks generally sit on or astride the party walls, they carry the rhythm of the roofline as much as the ridge does, and in a conservation area their removal is difficult to justify, because chimneys are among the character-bearing elements any roofscape argument is assessed against. Retention is therefore the working assumption, which has real consequences inside: the new stair, the new structure and the new floor all have to route around a breast and its flues rather than through them. Where a stack is shared, work to it is a party-wall matter as well as a planning one, so it belongs in the feasibility study, not the snagging list.
The choice between a rooflight scheme, a rear dormer and a mansard follows from those two facts. A conservation-style rooflight conversion leaves the roof form and the stepping ridge entirely intact and raises the fewest planning questions, but it adds the least usable head height and no floor area at the eaves. A rear dormer buys real floor space, and it is defensible when it is set down from the ridge, held in from the party walls and the flanks, kept below the point where it starts to compete with the stacks, and clad so it recedes against the existing slope. A mansard reshapes the roof outright, so it depends entirely on whether the roof and the street context can carry one; on a stepped terrace where the original roofline is legible, that is a high bar and we test it before designing to it.
Inside, the stair is what most often decides whether the project is worth doing. A narrow Victorian terrace has a stair core of fixed width, and the new flight has to rise within the going, rise and headroom limits of the building regulations, land with full head height at the top, and clear the chimney breast and the roof structure as it arrives. In practice it almost always takes space from a bedroom or landing on the storey below, and on a house that has already been through one conversion the obvious position may be occupied. We draw the stair at feasibility, in section as well as plan, because it determines the cost to the floor beneath, the usable area you actually gain in the roof, and in some houses whether the conversion is sensible at all.
Fire strategy is the requirement that reshapes schemes late if it is left late. Taking a two-storey house to three lifts the top floor above the 4.5-metre threshold and brings a higher standard with it: a protected stair enclosure running from the new floor all the way to a final exit, fire-rated doors — typically FD30 — to the habitable rooms opening off it, and mains-powered, interlinked alarms. Where a Victorian terrace has been opened up at ground floor, that enclosure often cannot be formed cleanly, and a sprinkler or mist system or a compliant alternative escape has to be designed in instead. In a converted flat or maisonette the problem is larger again, because fire separation and acoustic performance between dwellings, and the protection of the common escape route, all have to be evidenced.
Then there is the structure and the fabric it lands on. A converted roof needs new beams, typically bearing into the party walls of a terrace, designed and calculated by a structural engineer; the existing rafters, purlins and wall plates are older fabric and their condition governs how much of the original roof can be retained and where new loads can go. Because the roof is open at that point, this is when insulation, ventilation and the moisture behaviour of the build-up should be resolved together, in line with Camden's approach to designing retrofit rather than adding it. We take the roof from measured survey through the planning case and heritage statement to a building-regulations package a contractor can build from — and we do not promise approval, only a scheme argued on evidence.
