When can you not do a loft conversion?
The deciding factor is headroom: you need at least 2.2m from the top of the existing ceiling joists to the underside of the ridge. Below that, a standard conversion cannot retain the 2m of usable head height required once the new floor and insulation are installed.
Measure from the top of the ceiling joists — not the finished ceiling below — to the underside of the ridge timber. You need about 2.2m minimum. The new floor structure sits above the existing joists and takes 150 to 250mm, and insulation to the roof slope takes more, so 2.2m of raw height becomes roughly 2m of usable room. Below 2.2m, a conventional conversion will not produce a legal habitable room.
Roof structure is the second factor. Older Fulham terraces generally have a traditional cut roof with rafters and purlins, which converts easily because the space is open. Houses built or re-roofed from the mid-1960s onwards often have trussed rafters — a lattice of timbers filling the whole loft — which cannot simply be cut out. Converting a trussed roof needs a structural redesign and is significantly more expensive.
Where headroom is marginal there are still options, and it is worth knowing them before giving up. A mansard rebuilds the rear roof entirely and creates its own headroom, though it costs more and usually needs planning permission. Lowering the ceilings of the floor below is technically possible but disruptive and expensive, and it damages the proportions of the rooms underneath — we rarely recommend it.
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