Types of Loft Conversion — Which Is Right for Your Property?

A Velux (rooflight) conversion is the simplest type. No new structure is added to the roof — instead, roof windows are installed within the existing roof pitch. The roof shape does not change.

• Requirements:
• •Good existing head height (at least 2.2m from floor to ridge at the highest point)
• •Existing roof joists must be upgraded to a floor structure
• •Staircase must be added
• •Building Regulations required (but not planning permission — almost always within Permitted Development)

This is the least disruptive and least expensive conversion type. It is ideal for properties with good existing head height where the client does not need maximum space.

Typical cost: £18,000 – £30,000 Typical programme: 5–7 weeks on site

A rear dormer involves building a box-shaped extension to the rear slope of the roof. The dormer runs from ridge to eaves and from one side of the roof to the other (or close to it). It dramatically increases usable floor area and creates vertical walls and windows at the rear.

• Requirements:
• •Most commonly used on terraced houses in London
• •Usually within Permitted Development on terraced and semi-detached houses (dormer up to 40m³ for terrace, 50m³ for semi)
• •Building Regulations required (always)
• •Party wall notice required (wall to adjacent property)
• •Structural engineer coordinates the dormer structure and floor

This is by far the most popular loft conversion type in London. The flat roof dormer with Juliet balcony or French doors is a standard design in East and South London terraces.

Typical cost: £35,000 – £65,000 Typical programme: 8–12 weeks on site

A hip-to-gable conversion applies to properties with a hipped roof (where the roof slopes inward on all four sides, including the sides). The hip end is removed and replaced with a vertical gable wall, creating a much larger loft space. It is almost always combined with a rear dormer.

• Requirements:
• •Suits semi-detached and detached houses (not terraced houses, which typically have gable ends already)
• •Usually within Permitted Development for semi-detached properties (within volume limits)
• •Always requires planning permission if the property is on a corner plot or in a Conservation Area
• •Party wall notice required for the semi-detached neighbour
• •More complex structural work than a dormer alone

Typical cost: £45,000 – £75,000 Typical programme: 10–14 weeks on site

An L-shaped dormer combines a rear dormer with a hip-to-gable (or second dormer at the side). The result is an L-shape when viewed from above — creating maximum loft space on a terrace house. This is typically the maximum possible space on a standard terraced property.

• Requirements:
• •Most common on Victorian and Edwardian terraced houses with a rear-facing secondary roof structure (a back addition or 'outrigger')
• •Usually within Permitted Development for terraced houses with an outrigger
• •Party wall works on both sides in a terrace
• •More complex roofing work to achieve the L-shape junction watertight

Typical cost: £55,000 – £85,000 Typical programme: 10–14 weeks on site

A mansard conversion involves rebuilding the entire roof with a near-vertical rear slope (typically 70 degrees) and a flat top. This creates the maximum possible internal volume. Mansard conversions are common in Central and West London, particularly on Victorian terrace houses in conservation areas.

• Requirements:
• •Almost always requires full planning permission (even if not in a Conservation Area) because the roof shape changes fundamentally
• •In Conservation Areas, materials and design must be sympathetic to the area — usually grey zinc or lead
• •Very high-volume work — the entire rear half of the roof structure is rebuilt
• •More expensive than other types

Typical cost: £70,000 – £120,000+ Typical programme: 14–20 weeks on site

A complete loft conversion package from RCB includes:

• •Structural engineer design (floor structure, padstones, dormer frame)
• •Building Regulations application and inspections managed
• •Scaffold erection and strike
• •All roofing works (dormer structure, felt, battens, tiles/slates, lead flashings)
• •Roof window installation
• •New floor structure throughout
• •Staircase — full flight from first floor to loft
• •Internal walls (stud and plasterboard)
• •Insulation (roof, walls, floor)
• •First-fix electrical circuits (lighting, power, smoke alarms)
• •First-fix plumbing (en-suite waste and supply runs)
• •Velux windows or dormer windows
• •Plasterboard and skim throughout
• •En-suite bathroom installation (labour only — client supplies suite, tiles, fittings)
• •Fire doors to loft staircase

• Exclusions (standard position):
• •Second-fix materials (sanitaryware, tiles, light fittings, floor coverings)
• •Carpet — RCB does not supply or install carpet
• •Planning Application fee or LDC fee
• •Party wall surveyor fees
• •Architectural drawing fees (if not part of a design-and-build package)

Most loft conversions trigger associated works on the rest of the property:

• •En-suite bathroom — almost universal. The new bedroom needs its own shower room or bathroom.
• •New staircase — the loft needs a proper staircase. In many London terraces, this requires reorganising the first-floor layout.
• •First-floor refurbishment — with the scaffold up and trades on site, many clients refurbish the existing first-floor bathroom and landing at the same time.
• •Structural alterations — sometimes the existing first-floor layout needs a structural alteration (wall removal) to accommodate the staircase.
• •Electrical consumer unit upgrade — adding circuits for the new loft may require upgrading the distribution board.