Mansard Loft Conversions in London: A Complete Guide
A mansard loft conversion is the most extensive — and typically the most expensive — type of residential loft conversion. It involves rebuilding the rear (and sometimes front) slope of the roof at a near-vertical angle to create a flat-roofed upper storey with maximum floor area and head height. In central and inner London, mansard conversions are by far the most common choice, reflecting both the scale of the floor area gain and the planning policies in place across many London boroughs.
What a Mansard Conversion Involves
Unlike a rooflight conversion (which works within the existing roof structure) or a dormer conversion (which adds a box to the rear slope), a mansard conversion essentially replaces the rear portion of the roof with a new structure. The existing rafters are removed, a new rear wall is built at typically 70–72 degrees from horizontal (close to vertical), and a new flat roof is formed at the head. This creates a full-height space across the entire rear of the property with minimal slope and minimal wasted area.
The front slope of the roof is typically retained in a mansard conversion — maintaining the appearance from the street. At the rear, the flat roof section is typically finished with lead or an approved felt system, often with access for maintenance but not as an accessible terrace (which would require additional planning consent in most boroughs).
Planning Considerations
This is where mansard conversions differ fundamentally from other loft types in London. In most cases, a mansard conversion does NOT fall within Permitted Development rights — it requires full planning permission. However, in many London boroughs, mansard conversions have been so widely granted over the decades that a well-designed scheme to an established local precedent is routinely approved.
The key planning principles are: the rear slope must be set back from the rear elevation of the existing building (typically 200–300mm depending on the borough); the flat roof section must be set back from the front elevation; windows must be of appropriate size and pattern; and materials must complement the existing building.
Many London boroughs have Supplementary Planning Documents or design guides specifically covering mansard conversions — detailing the expected set-backs, window proportions, and construction standard. These are worth reading before appointing an architect.
Cost Range for Mansard Conversions in London
Single-width terraced house (standard mansard, rear only): £80,000–£130,000 for build-only works, including staircase, bathroom, and structural works.
Larger or wider properties: £120,000–£180,000+ depending on width, specification, and whether a front mansard element is also included.
Mansard conversions are more expensive than rooflight or basic dormer conversions because they involve replacing the entire rear roof structure and building a new masonry rear wall. The floor area gain is proportionally greater, which typically justifies the additional cost.
RCB builds mansard loft conversions across London — including planning coordination, structural engineering, and full principal contractor delivery.
07359 872594 | contact@rcbgroup.co.uk | www.rcbgroup.co.uk