Loft Conversions: Types, Planning, Costs & Drawings

Start with head height: measure from the top of a ceiling joist to the underside of the ridge. Then look at the roof structure — traditional cut rafters are usually easier to open up than modern trussed rafters — and where a new staircase could land on the floor below. If height is tight, a conversion type that reshapes the roof, such as a dormer, hip-to-gable or mansard, can create the space a rooflight-only conversion can't. Our measured survey confirms what's realistic for your house.

• Rooflight (Velux) — keeps the existing roof shape and adds windows in the slope; needs enough starting height • Dormer — extends the rear roof slope out to create vertical walls and more floor area • Hip-to-gable — replaces a sloping side (hip) with a vertical gable, common on semis and detached houses • L-shape dormer — two dormers joined over the main roof and back addition, typical on terraces • Mansard — rebuilds the rear slope almost vertically for near full head height; usually needs more planning scrutiny Each type has its own guide, linked below.

Many house loft conversions in England can be carried out under Permitted Development, provided the Class B limits and conditions are met — including volume allowances of 40m³ for terraced houses and 50m³ for other houses, which count previous roof enlargements. Rooflights may fall under Class C. Flats and maisonettes, conservation areas, Article 4 areas and enlargements on a principal elevation fronting a highway usually need a householder planning application. Where Permitted Development applies, a Lawful Development Certificate gives formal evidence from the council that the proposal is lawful based on the information submitted.

Whatever the planning route, a habitable loft must meet the Building Regulations: fire safety and means of escape, stairs, insulation, ventilation and sound. Building Regulations drawings set out how the design achieves this. Structural calculations size the new floor joists, steel beams and roof alterations. They're prepared by a structural engineer, coordinated with the drawings and submitted for building-control review.

1. Survey — the house and roof are measured 2. Design — existing and proposed drawings are prepared and refined 3. Planning — an LDC or planning application is submitted where needed 4. Technical — Building Regulations drawings and structural calculations are completed 5. Building control — an application is made and inspections are arranged during the work 6. Build — your chosen builder carries out the construction Party wall matters, where they apply, run alongside as a separate process.

Build cost depends mostly on the conversion type, size, roof structure, stair position, bathrooms and specification, plus location. Design and council fees are separate from the build. Our loft conversion cost guide gives typical ranges for each type.

Loft conversion drawings can be prepared by an architect, an architectural designer or technician, or a specialist loft-design service. "Architect" is a protected title in the UK. Our guide to whether you need an architect explains the differences neutrally so you can choose the right fit for your project.

We look after the design and approval stage: measured survey, planning or LDC drawings, Building Regulations drawings, structural calculations and the council submissions. Planning drawings start from £950 and the full package from £1,500. You then take the completed pack to the builder of your choice — see our portfolio for real projects we've designed.

Frequently asked questions

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