Loft Conversions: Everything You Need to Know

A loft conversion is one of the most popular and cost-effective ways to add living space and value to a UK home. Converting an unused roof space into a bedroom, bathroom, office, or studio can add 15-25% to a property's value — and for most houses, it falls within permitted development rights.

The most common types are: a Velux (rooflight) conversion — the simplest and cheapest, adding windows to the existing roof without altering its shape; a dormer conversion — adding a box-shaped extension to the rear roof slope, the most common type for semi-detached and terraced houses; a hip-to-gable conversion — extending the sloping hip end of a roof to a flat gable, common on detached and semi-detached houses; and a mansard conversion — a major structural change creating nearly vertical rear walls, maximising headroom.

Most loft conversions fall under permitted development — meaning no planning permission is needed — provided they meet conditions: maximum volume addition of 50 cubic metres for detached houses or 40 cubic metres for semi-detached and terraced; no extension beyond the highest point of the existing roof; no side-facing windows within 1.7 metres of the floor (to protect privacy); and materials that are similar in appearance to the existing house. Mansard conversions and anything on principal elevations usually require full planning permission.

All loft conversions require Building Regulations approval, regardless of whether planning permission is needed. Building control will check: structural calculations (joists, beams, and wall loads); fire safety — a protected means of escape, typically requiring self-closing fire doors on each landing and mains-wired smoke alarms; insulation to Part L standards; and staircase design meeting minimum headroom and angle requirements.

The fire safety requirements for loft conversions are the most commonly underestimated aspect by homeowners. A 30-minute fire door on the ground floor is not a cosmetic upgrade — it is a legal requirement.

— National House Building Council (NHBC)

A Velux conversion typically costs £15,000-£25,000. A standard dormer conversion runs £30,000-£50,000. A hip-to-gable conversion is typically £35,000-£55,000. A full mansard conversion can reach £60,000-£80,000 or more. These are guide figures for England; costs are higher in London. Always include contingency (10-15%) for unforeseen structural issues — particularly in older houses where the roof structure may not be what drawings suggest.