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Conversions

Loft conversion estimate.

Loft conversions are one of the most cost-effective ways to add habitable space. This guide covers the main conversion types, what to price, and the structural and regulatory details builders often miss.

Authored by Alexandr Vreme & Maksym Vasylkov
Published July 2026 — Next review July 2027

Who this guide is for

For builders and estimators pricing loft conversions of all types — Velux, dormer, hip-to-gable, and mansard. Each type has different cost drivers, and the choice depends on the existing roof structure and the client’s budget.

Information you need before pricing

• Existing roof structure — trussed rafters or traditional cut roof?
• Available headroom — is there enough height for a habitable room without raising the roof?
• Desired conversion type — Velux, dormer, hip-to-gable, or mansard
• Existing ceiling joist sizes — will they support a floor load, or do they need upgrading?
• Chimney stack location — may need trimming around or removing
• Water tank location — typically in the loft; removal and re-routing required
• Access for the staircase — landing space and headroom on the floor below

Types of loft conversion

• Velux (roof-light) — most economical. Roof windows only, no structural roof alteration. Suitable where headroom already exists.
• Dormer — most common. A box projection from the roof slope creating headroom and floor space. Flat-roof dormers are standard; pitched-roof dormers are more expensive.
• Hip-to-gable — for end-of-terrace and semi-detached houses. Replaces the sloping hip with a vertical gable end, gaining significant internal space.
• Mansard — alters the entire rear roof slope to a near-vertical pitch. More complex and expensive but maximises floor area.

Typical scope of works

• Structural works — steel RSJs to support altered roof and floor loads
• New floor joists — upgrading existing joists or installing new ones
• Staircase installation — new flight from the landing, with handrails and fire regulation compliance
• Dormer construction (if applicable) — new walls, roof covering, windows, and cladding
• Roof windows — Velux or similar, flashed into the roof covering
• Insulation — between and over rafters to meet building regs U-values
• Plasterboard and skim finish to walls and ceilings
• First and second fix — electrics, lighting, heating, and sockets
• Fire doors and escape route — upgrading doors and creating a protected route

Commonly missed costs

• Structural engineer report and calculations for steel and floor loading
• Building regulations for fire escape — may require a protected stairway and upgraded doors
• Upgrading existing smoke alarms — linked alarm system across the property
• Making good ceilings and decoration below the staircase opening
• Party wall consent for semi-detached or terraced properties
• Scaffold hire for roof access and dormer construction

Miniature worked example

A Velux loft conversion on a 3-bed semi-detached house:

• Structural works (steel, new floor joists): £8,000
• Staircase installation and landing alterations: £4,000
• Dormer element (walls, roof, cladding, windows): £12,000
• Internal fit-out (insulation, plaster, first and second fix, decoration): £18,000
• Staircase hall alterations and making good below: £3,000

Total: approximately £45,000 for a Velux conversion. A dormer variant on the same property would be in the region of £55,000. Prices vary by region, roof complexity, and specification.

Suitable output documents

• Detailed estimate separating structural works from fit-out
• Priced bill of quantities with staircase and fire-regulation items
• Schedule of works with sequenced trades and building control hold points

Questions to ask before pricing

• Cut roof or trussed rafters? Trusses need structural alteration, adding cost.
• Is the existing ceiling strong enough, or do joists need upgrading?
• Where will the staircase land on the floor below — can a straight flight fit?
• Does the property have a party wall that affects the conversion?