A rear mansard replaces the rear slope of your roof only — the street-facing elevation remains unchanged. A double (or front and rear) mansard replaces both slopes, creating a complete new storey across the full property footprint. A double mansard adds more space but requires more complex planning permission, as it alters the street elevation.
Yes, for terraced and semi-detached properties. The Party Wall etc. Act 1996 requires notices to be served on adjoining neighbours before any structural changes near the shared wall. Simply Loft coordinates this as part of your project. For full details visit our party wall guide.
Most London pitched-roof houses are suitable. The key requirement is a minimum finished ceiling height of 2.2 metres. You can self-check by measuring from the loft floor to the highest point of the roof space. Pre-1960s properties with traditional cut rafters are most straightforward; post-1960s trussed roofs require additional steelwork but are still convertible. A Simply Loft surveyor will assess suitability at no cost and no obligation.
A Permitted Development dormer typically takes 4–7 months from survey to completion: design 2–4 weeks, party wall 6–8 weeks (runs in parallel), construction 8–14 weeks. Add 8–13 weeks if full planning permission is needed. Simply Loft provides a project-specific timeline estimate at the survey stage
Flat roof rear dormers: £45,000–£60,000. Hipped roof dormers: £48,000–£65,000. Gable fronted dormers: £40,000–£58,000. All prices are fully inclusive of design, building regulations, and build. Simply Loft quotes are fixed-price after survey.
An Lawful Development Certificate (LDC) is a formal document from your local planning authority confirming your conversion was lawfully carried out under Permitted Development. It is not legally required to build the conversion — but it provides legal protection if any dispute arises and reassures future buyers. Simply Loft recommends applying for an LDC for all Permitted Development dormer projects.
Most rear dormer conversions in London do not require planning permission under Permitted Development rights. The main limits are 40 cubic metres total added volume for a terraced house, or 50 cubic metres for semi-detached or detached. Front dormers always require full planning permission. Properties in conservation areas and listed buildings are not covered by Permitted Development. Simply Loft confirms your planning route at the free survey stage.
The main types are: flat roof dormer (most space, most common in London), shed roof dormer (gently sloped, suits gable-ended houses), L-shaped dormer (ideal for Victorian terraces — see our dedicated L-shaped dormer page), gable fronted or doghouse dormer (pitched roof, suits traditional architecture), and hipped roof dormer (sloped on three sides, preferred in conservation areas). Simply Loft designs and builds all types across London.
A new build loft conversion can cost up to £40k and over depending on the complexity and scale of the project involved. We can quickly and easily assess your needs and the project requirements. Get a Free Quote.
A mansard loft conversion is built to the rear of a property, altering the slope of the roof to a 72 degree gradient. They are sympathetic to the existing architecture of the property, and therefore are preferable on homes in conservation areas and heritage zones. Read more on the popularity of mansard loft conversions.
Book a free, no-obligation site survey and one of our surveyors will assess your loft, talk through your options and show you examples of our work.
