Costs, types and planning
Explore what your roof could become, what different conversions cost and how to plan your next step.
The direct answer
Indicative guidance including VAT runs from £50,000 – £85,000 for a roof-light conversion to £85,000 – £150,000+ for a mansard. The roof structure, staircase, rooms and finish decide where your home sits within the range.
Verified project proof
The verified project record documents a loft conversion adding bedroom accommodation alongside a side extension, double-storey rear extension and full internal refurbishment.
Explore the loft conversion as part of this Isleworth renovation and extension project.
View the verified Isleworth project
Suitability first
The existing roof form and timbers determine how the new floor and roof can be supported.
Measure usable standing space after allowing for the new floor, insulation and finished ceiling.
The stair must arrive with safe headroom without taking more space than necessary from the floor below.
Bedrooms, storage and an en-suite each affect drainage, daylight, fire safety and the final layout.
Compare how each option changes your roof and the space inside.
What changes: A full-width box projects from the rear roof slope, creating upright walls and more full-height floor area.
Best suited to: Terraced and semi-detached homes where useful room width matters.
Rear dormer design and planning guide
What changes: The sloping side of a hipped roof is built up into a vertical gable, widening the space inside.
Best suited to: Many semi-detached and end-of-terrace homes with a hipped side roof.
Hip-to-gable suitability guide
What changes: A roof slope is rebuilt as a steep face with a shallow top, creating a regular room across more of the footprint.
Best suited to: Period terraces where the design and planning context support a larger roof alteration.
Mansard design and approvals guide
What changes: The main roof profile stays in place while windows are fitted into the existing slope.
Best suited to: Roofs that already have enough usable height and floor area without a dormer.
Compare roof-light and dormer optionsGoverned cost guidance
These indicative ranges provide an early budget position for a standard specification. They are not a formal quotation.
Typical project investment: approximately £60,000-£100,000 including VAT.
Use the London cost calculatorSome loft conversions can proceed under permitted development if the proposal meets all relevant limits and conditions and those rights apply to the property. Flats and maisonettes have different rules. Designated land, including conservation areas, and listed buildings carry additional restrictions, so confirm the permissions and consents required for your property with the local planning authority.
Building Regulations apply separately, and Party Wall matters may affect terraced and semi-detached homes.
Government permitted development technical guidanceA property-specific programme is set once the design and approvals route are understood.
Your next step
Tell us where the property is and what you hope to create. We’ll respond within 24 hours.
Use the short enquiry above for your home, or explore how Trovix approaches loft conversion design and delivery.