What makes a mansard different?
Unlike a dormer, which projects a flat-roofed box from the existing pitched roof, a mansard rebuilds the entire roof structure. The party walls are raised, a new vertical rear wall is built in brick or block, and a steep slate-clad slope (typically pitched between 70 and 72 degrees) rises from a parapet at the front to a small flat ridge at the rear. Internally this gives you the full plan area of the floor below — there are no sloping ceilings eating into the corners. The mansard is the only loft type that genuinely creates a full additional storey, which is why estate agents value mansard-converted terraces at 25%+ more than equivalent unconverted properties in central London. The construction is heavier and more involved than a dormer, requiring deeper structural steels, raised party walls, and full planning permission in almost every case, but the result is a finished space that feels like a permanent part of the original house rather than an addition.
Where mansards are required (and where they shine)
Across Camden, Islington, Hackney, Westminster, Kensington and Chelsea, Hammersmith and Fulham, Wandsworth and Lambeth, large parts of the borough fall within conservation areas or Article 4 directions. Local plans in these areas generally refuse rear dormers (considered too modern) and instead require any roof extension to match the historic Parisian-style mansard roofscape. If your house is on a Victorian or Georgian terrace within a conservation area, a mansard is usually the only loft type your borough's planning department will approve. We handle the full planning submission including design and access statement, heritage impact assessment, and pre-app consultation with the borough conservation officer — included in our fixed-price quote rather than billed separately. Our planning success rate on mansards in conservation areas is over 90% because we know what each borough wants before we draw a line.
Structural and party wall considerations
A mansard is structurally more demanding than a dormer. The party walls need to be raised in brick or block, requiring full party wall agreements with both neighbours under the 1996 Party Wall Act — we serve notice on your behalf and instruct a single agreed surveyor where possible to keep costs down. The new floor is supported on heavy structural steels (typically 203 UC sections) bearing onto pad-stones on the party walls. The front parapet and rear vertical wall are tied back to the existing structure. A structural engineer attends site multiple times during the steel erection and slate roof installation. Crucially, the mansard's weight needs to be assessed against the original foundations — on Victorian terraces these are usually corbelled brick footings around 300–450mm deep, which our engineer confirms are adequate via trial pit excavation before any work starts. If foundations need underpinning to take the additional load (rare but possible on poorer subsoil) we cost that transparently as a separate scope.
Materials and finishes
The external slope is traditionally clad in natural Welsh slate fixed to ventilated battens over a breathable membrane. We use Penrhyn or Cwt-y-Bugail slate at 500x250mm or 600x300mm depending on the borough's preference. Lead flashings and rolls are used throughout. For projects where the conservation officer prefers a more contemporary expression we offer zinc standing seam or pre-weathered lead cladding — both increasingly approved in Hackney, Tower Hamlets and parts of Lambeth. Internally, mansards almost always feature solid oak engineered flooring, plastered finishes with traditional cornicing where appropriate, and bathroom specs that match or exceed the principal floor below. We work with three London bathroom showrooms (Lusso, CP Hart and West One Bathrooms) and quote a transparent PC allowance that lets you specify exact products without surprise extras.
Timeline expectations
From first survey to handover, a mansard typically runs 6–9 months total. Design and drawings take 4–6 weeks. Full planning permission averages 8–12 weeks in most London boroughs. Party wall agreements run in parallel and complete in 4–6 weeks. Building control plans approval and structural calculations finalise during planning. Site duration is 12–16 weeks for a standard mansard, longer for larger or double-fronted properties. We hold weekly progress meetings, post photo and video updates to a private client portal, and never start a mansard without all approvals in place — false starts cost everyone money.
Living through a mansard build
Mansards are more disruptive than dormers because the entire roof comes off — typically over a 2–4 day window — and a temporary roof is erected over the property while the new structure is built. Most clients decant for the demolition and roof phase (about 2–3 weeks) and return for the fit-out. We can recommend short-let agencies in your borough and structure our schedule to minimise the decant window. If you do stay, the rooms below are protected with crash decking and dust sheeting, and the existing stairs are sealed off from the loft level until plaster is complete.
Investment case
On a £1.2M Camden terrace, a £130k mansard typically adds £250–350k of value, with the upper bedroom suite becoming the property's primary bedroom and the original bedrooms repurposed as children's rooms, guest accommodation or studies. On Wandsworth and Clapham four-bed terraces sitting in the £1.5–2M range, mansards are now expected by buyers — the absence of one is a discount, not a value-neutral. We share past mansard valuation data with every prospective client during the consultation so you can model your specific property against comparable recent transactions.
