The honest answer is: it depends on the type. Below is a summary of average costs for the four main loft conversion types in 2026.

These figures reflect a standard specification including structure, staircase, insulation, and basic internal finish. They exclude high-end fixtures, bespoke joinery, and en-suite bathrooms. Location also plays a significant role – costs in London and the South East run 15-25% higher than the national average.

A Velux conversion is the simplest and most affordable option. It keeps the existing roof structure entirely intact, adding only roof windows to let in light and provide ventilation. Because there are no structural changes to the roofline, it falls under permitted development in most cases and avoids the need for full planning permission.

This makes it the fastest and least disruptive conversion type – most projects complete in six to eight weeks. It works best in properties that already have good ridge height (typically 2.2 metres or more between the floor and the ridge).

• Best for: properties with adequate existing headroom and a tight budget.
• Not suitable for: lofts with limited height, or where you need a bathroom with a shower.
• Watch out for: trussed roofs – these require significant structural modification and can push costs well above the standard Velux range.

The dormer is the most popular loft conversion type in the UK. It involves constructing a box-shaped extension that projects vertically from the pitched roof, creating a flat ceiling and straight walls that dramatically increase usable floor space and headroom.

A rear dormer is the most common configuration, typically adding a full-height room across the back of the property. It suits the widest range of property types and, in most cases, falls under permitted development.

For a standard 25-35m2 rear dormer in 2026, expect to pay £50,000-£65,000. This usually includes the structural shell, staircase, insulation, plasterboard, and basic decoration. Getting a detailed build cost estimate from plans before going to tender is strongly recommended.

• Best for: most property types; excellent balance of cost, space, and planning simplicity.
• Popular combination: rear dormer with Velux windows to the front for maximum light.

Hip-to-gable conversions are suited to semi-detached and detached properties with a hipped roof – one that slopes inward on all four sides. The conversion replaces the sloping hip end with a vertical gable wall, substantially increasing the volume of the loft space.

This is almost always combined with a rear dormer to make the most of the space created. The structural complexity involved means costs are higher than a standard dormer. Because the roof shape is being fundamentally altered, planning permission is usually required.

• Best for: semi-detached or detached homes with hipped roofs wanting maximum space.

Planning note: planning permission is typically required due to roof shape changes.

A mansard conversion involves a complete restructuring of the roof – raising the party wall and creating an almost vertical back wall (typically at a 72-degree angle) with a nearly flat roof. The result is the largest possible internal volume of any conversion type.

Mansard conversions are particularly associated with Victorian terraces, especially in London. They almost always require planning permission, and because of the structural complexity involved, they also take the longest to build.

• Best for: terraced properties (especially Victorian) wanting the maximum possible space.
• Important: planning permission required in almost all cases.