Loft ConversionCost Calculator

Loft conversion costs in the UK in 2026 range from £20,000 to £80,000+ depending on the type. A basic velux/rooflight conversion costs £20,000–£35,000. A rear dormer conversion — the most popular type — costs £35,000–£55,000. A hip-to-gable conversion (often combined with a dormer) costs £40,000–£60,000. A mansard conversion — which involves major roof remodelling — costs £55,000–£80,000+. London prices are typically 25–35% higher than the national average.

Most loft conversions fall under Permitted Development (PD) rights and do not require planning permission, provided they meet conditions: the conversion must not exceed 40 m³ additional roof space (50 m³ for detached houses), no extension beyond the existing roof plane at the front, and materials must be similar in appearance to the existing house. Mansard conversions almost always require full planning permission as they significantly alter the roofline. Properties in conservation areas or listed buildings require consent for any roof alterations.

An en-suite is not required but is highly recommended for a loft bedroom — the cost to add one during construction (£3,000–£6,000) is much lower than retrofitting later. An en-suite significantly increases the value added by the conversion. For a bedroom-only loft without an en-suite, consider at minimum running the waste and supply pipes to the loft during construction, so a bathroom can be added later at lower cost. A velux conversion without an en-suite is the most affordable entry point.

A loft conversion typically adds 15–25% to a property's value in the UK. On a £350,000 house that's £52,500–£87,500 of added value. A dormer conversion with en-suite converting a 3-bed to a 4-bed home is typically the sweet spot for ROI. Returns are strongest in London and the South East where the value per bedroom is highest. Mansard conversions in London can add £100,000+ in value but cost significantly more to build. Always commission an independent valuation before and after to measure the actual uplift.

The minimum habitable head height recommended for a loft conversion is 2.2m at the ridge (highest point). The usable floor area is typically measured where the ceiling height exceeds 1.5m. A velux or rooflight conversion requires sufficient existing ridge height — if your loft measures less than 2.2m at the ridge, a dormer or mansard conversion will be needed to create usable space. Your architect or loft conversion specialist can assess head height during a free survey.

Yes — all loft conversions require Building Regulations approval regardless of whether planning permission is needed. Building Regulations cover structural stability (floor joists, roof modifications), fire safety (escape windows, fire doors, protected staircase), thermal insulation, sound insulation between floors, and electrical and plumbing work. Your loft conversion builder will apply for Building Regulations approval as part of the project and arrange inspections at key stages. A completion certificate is issued when the work is signed off — retain this for when you sell the property.