Most Marylebone roof conversions come down to a choice between three forms. A mansard rebuilds the rear, and sometimes front, roof slope to a steep near-vertical pitch, giving close to full head height across the plan; a dormer keeps the existing pitch and pushes out a box to gain standing room over part of the floor; a rooflight scheme leaves the roof profile untouched and simply brings light into whatever head height already exists. Which of these is realistic depends as much on the roofscape Westminster is protecting as on what your roof timbers will physically allow.
Head height is the first thing worth measuring. A habitable room needs usable standing space over a meaningful proportion of its floor, and many Marylebone terraces have a shallow front pitch that gives very little of it without a dormer or mansard. The existing ridge height, the pitch of the front and rear slopes, and the depth of the structural floor you have to build up all set the ceiling — quite literally — on what the roof can become. We measure these before discussing layouts, because a plan drawn to the wrong head height is wasted effort.
The existing roof structure rarely carries a new floor without help. Most period roofs were framed to hold up slates and tiles, not to support a bedroom, so a converted roof usually needs new floor joists spanning onto the party walls, or new steel beams, with the loads traced down through the building. Insulation matters too: bringing a cold roof void up to current thermal and fire standards while keeping enough head height is a genuine constraint in a shallow Marylebone roof. These are engineering questions we resolve with a structural engineer before the drawings are fixed.
A new storey needs a new stair, and in a tall Marylebone townhouse that stair has to do two jobs. It has to land sensibly within the floor below without sacrificing a whole room, and it has to form part of a protected escape route. Once a converted floor sits more than 4.5 metres above ground, Building Regulations generally require a protected stair enclosure — fire-resisting doors and partitions giving a safe way down to the final exit. On upper mansion-flat levels the escape strategy can be more involved still, and it shapes the layout from the outset.
How the roof reads from the street is decisive here. Marylebone's estate streets were laid out with deliberately even rooflines, and Westminster resists roof additions that break the rhythm of a uniform terrace or interrupt a protected roofscape. A mansard is often most defensible where an established pattern of mansards already runs along the terrace; an unbroken frontage may only suit concealed rooflights. Where the Howard de Walden or Portman estate is the freeholder, an estate surveyor may review the roofline alongside Westminster, and a leaseholder will usually need a Licence to Alter before work starts.
None of this can be judged from a postcode alone. The right roof form depends on the specific terrace, its planning and alteration history, the estate context and what survives of the original roof. We start by confirming the conservation area and freehold position for the address, then test the head height and structure, so the scheme we draw is one that Westminster, the estate and Building Regulations can all realistically accept. We never promise a particular planning outcome, but we make sure the case put forward is the strongest the roof can support.
