What makes a loft suitable for conversion?
Four things decide it, and head height is the one that kills most projects — it is worth measuring before spending anything on drawings.
Head height. Measure from the top of the existing ceiling joist to the underside of the ridge. You lose height at both ends: a new floor structure is built above the existing joists, and insulation plus plasterboard comes below the rafters. As a working guide, roughly 2.2–2.4 metres of existing height is usually needed to end up with a usable room, though the exact figure depends on the construction. Building regulations also set a minimum headroom over the staircase, and that requirement, rather than the room itself, is frequently what makes a conversion impossible.
Roof structure — the single biggest cost variable.
Traditional cut roof — rafters and purlins, common in houses built before around the mid-1960s. Open space beneath, straightforward to convert.
Trussed rafter roof — the W-shaped factory-made trusses used in most houses since. The bracing runs straight through the middle of the space. Trusses must never be cut without an engineered solution: a new structural floor and often steel beams are installed to carry the load before any truss member is removed. Convertible, but considerably more expensive.
Roof pitch. A steeper pitch gives more usable volume at a given footprint. A shallow pitch may need dormers or a roof redesign to produce anything worth having.
Structure below. The new floor and its loading must be carried down to adequate support, which is why a structural engineer's calculations are part of every conversion.
Other practical constraints: where the staircase will go, and what it costs you in the room below — usually the second-hardest problem; fire safety, which is why a loft conversion triggers requirements for protected escape routes, fire doors and interlinked alarms; and water tanks, chimneys and services to be relocated.
Permitted development frequently covers the works, with volume limits and restrictions on front-facing dormers, but building regulations approval is always required.