Loft Conversion Cost Calculator UK

Loft conversion costs vary widely by type: Velux (rooflight only) £20,000-£35,000, rear dormer £35,000-£55,000, hip-to-gable £45,000-£65,000, mansard £55,000-£75,000, and L-shaped dormer £50,000-£70,000. These are national averages — London adds 25-30%. Add-ons like en-suites and staircase relocation increase costs further.

A Velux conversion adds roof windows to the existing roof structure without changing the roofline — it is the cheapest and quickest option but gives limited headroom. A dormer extends out from the roof slope, creating a box-like structure with full-height walls and significantly more usable floor space. Most 3+ bed homes opt for dormers for the extra space.

Most loft conversions fall under Permitted Development rights and do not need planning permission, provided you stay within limits: up to 40m³ for terraced houses or 50m³ for detached/semi-detached, no extension higher than the existing roof, no veranda or balcony, and materials matching the existing house. Mansard conversions and conversions in conservation areas usually need planning permission.

All loft conversions require Building Regulations approval covering: structural integrity (floor joists, steels), fire safety (fire doors, escape windows, smoke alarms, 30-minute fire resistance), staircase design (headroom, width, pitch), insulation (thermal and sound), electrical work, and ventilation. A Building Control officer must inspect and sign off the work.

A hip-to-gable conversion extends the sloping side wall (hip) of the roof upward to create a vertical gable wall. This is common on semi-detached and detached houses where the side roof slopes inward, wasting potential loft space. It creates significantly more usable floor area than a dormer alone and is often combined with a rear dormer for maximum space.

A Velux conversion takes 4-6 weeks. A standard rear dormer takes 6-8 weeks. Hip-to-gable and mansard conversions take 8-12 weeks. L-shaped dormers take 8-10 weeks. These are build times only — add 4-8 weeks for architectural drawings and structural calculations, plus 6-8 weeks for Building Regulations and any planning applications.

If your loft conversion involves work on or near a shared wall (terraced or semi-detached houses), you need a Party Wall Agreement under the Party Wall etc. Act 1996. This applies to: cutting into the party wall for steel beams, building on the boundary, or excavating near the neighbour's foundations. You must serve notice at least 2 months before work begins. A party wall surveyor costs £800-£1,500 per neighbour.

A loft conversion adding a bedroom with en-suite typically adds 15-20% to property value. Dormers and hip-to-gable conversions offer the best value-add because they create the most usable space. A master bedroom suite with walk-in wardrobe and en-suite is the most desirable outcome. In high-value areas, the return on investment can exceed 2:1.

Not all lofts are suitable. The key requirement is sufficient head height — ideally 2.2m+ from the top of the ceiling joists to the underside of the ridge beam. The roof structure matters: traditional cut timber roofs are easier to convert than modern trussed roofs (which need significant structural work). A surveyor or loft specialist can assess suitability in a free or low-cost survey.

A new staircase must comply with Building Regulations: minimum 2m headroom, treads at least 220mm going, maximum 42-degree pitch for a primary staircase. The staircase usually takes space from an existing bedroom or landing. Alternating tread stairs may be permitted where space is very tight but are not ideal. Staircase positioning is one of the most important design decisions.