Mansard Loft Conversion — The Ultimate in London Loft Space
A mansard loft conversion creates near-vertical rear walls (typically at 72° from the horizontal) and a flat or gently sloping roof, maximising floor area and headroom across the entire width of the property. It is the most extensive — and most visually dramatic — form of loft conversion available, and it is the type most associated with the classic London period townhouse.
Unlike a dormer, which extends a section of the roof, a mansard effectively rebuilds the entire rear roofline. The result is a full additional storey that feels like a natural part of the building rather than an extension. Combined with floor-to-ceiling glazing or Juliet balconies, a well-designed mansard creates a genuinely stunning living space.
The Benefits of a Mansard Conversion
Maximum Floor Area
Near-vertical walls mean almost every square centimetre of roof space becomes usable floor area. There are no sloped ceilings cutting into the room.
Full Headroom Everywhere
Unlike a dormer, where headroom tapers at the sides, a mansard provides full 2.4m+ ceiling height across the entire floor area.
Premium Aesthetics
The mansard roofline is the signature look of London’s finest period conversions — architecturally considered and visually impressive from both inside and out.
Best Return on Investment
In inner London — Islington, Chelsea, Hackney, Brixton — a mansard conversion consistently delivers the highest absolute value uplift of any conversion type.
Does a Mansard Require Planning Permission?
Almost always, yes. Because a mansard changes the entire rear roofline rather than adding a dormer to an existing roof, it almost universally falls outside Permitted Development and requires a full planning application. This means engaging with your local authority, producing professional planning drawings, and in many cases consulting with a conservation officer if your property is in a conservation area.
Prime Loft’s in-house planning team manages the full application process. We have a strong track record of securing consent for mansard conversions across all inner London boroughs, including within conservation areas where the bar is higher.
Full Planning Guide →Mansard Loft Conversion — FAQs
Mansard conversions in London typically range from £52,000 to £90,000+ depending on the size of the property, specification, and planning complexity. Central London and conservation area projects command a premium. All Prime Loft prices are fixed.
In almost all cases, yes. The scale of change to the roofline takes a mansard outside Permitted Development. We manage the full planning application as part of your fixed price.
The build phase typically takes 8–12 weeks. The pre-construction phase — design, planning application, Building Regulations approval — typically adds 12–16 weeks, longer if planning is complex.
The top-floor flat of a mansion block or converted house can potentially have a mansard-style conversion, but this requires the freeholder’s consent and usually a licence to alter. Our team handles the full legal and planning process for these projects.
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