Mansard Loft Conversion DrawingsFrom £845
Does a mansard loft need planning permission in London?
Yes, in London a mansard loft conversion almost always needs a householder planning application, because it rebuilds a roof slope instead of adding a box to it. Our mansard planning drawings start at £845 with the measured survey included, and the council's own fee and the other costs are set out below. The rest of this page explains which permitted development tests a mansard fails, the one route that sometimes gets close, and what London councils look for when they decide a mansard application.
Why a mansard is a planning application, not permitted development
A mansard replaces a pitched roof slope with two new ones: a steep lower slope that is nearly a wall, with the windows set into it, and a shallow or flat top running back to the ridge. The National Planning Policy Framework's own glossary describes it as a roof characterised by two slopes, the lower steep and the upper shallow. That change of shape is the whole point, because it turns a triangle of loft into a room with near-full head height across most of its depth. It is also why the planning position is different from a dormer.
Permitted development for roofs was written for additions to a roof that stays recognisably the same: a dormer on the back slope, a hip turned into a gable, roof lights. A mansard takes the old slope away. On almost every London house that puts it outside the permitted development right and into a full householder application, decided by the council against its local plan and design guidance. If you are still weighing up the type of loft, our post on what drawings each kind of loft conversion needs compares the routes side by side; this page stays with the mansard and the planning question.
The Class B tests a mansard runs into
Roof extensions to houses are permitted development under Class B of Schedule 2, Part 1 of the General Permitted Development Order 2015. Three of its limits matter most for a mansard. Nothing may extend beyond the plane of an existing roof slope that forms the principal elevation and fronts a highway, so a front mansard on a street-facing slope fails straight away. The extra roof space on a terrace house is capped at 40 cubic metres, or 50 cubic metres on a semi or detached house, and a full-width rebuild of the back slope uses that allowance quickly.
The condition that settles it for most rear mansards is about the eaves. Class B requires the eaves of the original roof to be kept or reinstated, and the edge of the new work nearest the eaves to sit, so far as practicable, at least 0.2 metres from them, measured up the slope. A true mansard's steep slope rises from the back wall or parapet, so there is no original eaves line left to keep and no 0.2 metre strip in front of it. That is why some builders offer a slate-hung rear dormer with a steeply angled face, set back from the eaves and inside the party walls: it is a dormer shaped like a mansard. It can be permitted development where every Class B limit is met, and we apply for a lawful development certificate, from £645, so the council confirms it before anyone builds.
- Front slope
No work beyond a roof slope that forms the principal elevation and faces a highway, so a street-side mansard is out.
- Eaves
The original eaves stay, with the new work about 0.2 m up the slope from them. A slope rising off the back wall removes them.
- Volume
The added roof space stays within 40 cubic metres on a terrace or 50 cubic metres on other houses.
- Location
Class B does not apply on article 2(3) land, which includes every conservation area in London.
Conservation areas, and the 1948 cut-off for upward extensions
Class B is not available at all on article 2(3) land, and conservation areas are part of that land. A large share of London's Victorian and Georgian terraces sit in conservation areas, so for those houses any roof extension, mansard or dormer, goes through a planning application regardless of size. An Article 4 direction or a listed building takes the rights away too, and a flat has no householder permitted development rights to begin with.
There is a second permitted development right that people sometimes point to: Class AA, which allows a house to be enlarged by building up to two additional storeys on top, subject to prior approval from the council. It was designed for exactly this kind of upward extension, but the Class AA text excludes any house constructed before 1 July 1948 or after 28 October 2018, and any house on article 2(3) land. Most London houses suited to a mansard are Victorian or Edwardian, so Class AA does not reach them. For a post-war house outside a conservation area it can be worth testing, and we check the build date before recommending a route.
What national planning policy now says about mansards
The August 2026 National Planning Policy Framework names mansards directly. Its policy on making effective use of land says substantial weight should be given to the benefits of creating additional homes or floorspace within settlements by using the airspace above existing residential premises, and it names the addition of mansard roofs as an example. Such proposals should be consistent with the overall street scene, keep acceptable daylight, sunlight and privacy for neighbours, and maintain safe access.
The same document's glossary says a mansard is generally regarded as a suitable roof extension for buildings that are part of a terrace of at least three buildings, at least two storeys tall, with a parapet running the length of the front facade. In practice that means a mansard on a parapeted London terrace starts from a position national policy supports. It does not mean every mansard gets consent: the council still weighs the design against its own local plan, conservation area appraisals and design guide, and a mansard that breaks the rhythm of a street with unaltered roofs can still be refused.
How London boroughs judge a mansard: Lambeth's numbers as an example
Each borough publishes its own roof guidance, and the detail is what officers comment on. Lambeth's Design Guide Part 4 (August 2023) is a clear example. For a rear mansard it asks for the lower pitch to be 70 degrees and hung with slate, or clay tile where that is the host roof's material, and for the 70 degree slope to stop at 2.3 m above the floor where the mansards along a row are meant to link up. It says there must be sufficient internal headroom of 2.1 m below the existing ridge, or the principle of a rear mansard will not be acceptable. That 2.1 m test is the first thing we measure in the survey.
The same guide asks for the party wall upstands to be extended in matching brick with a brick-on-edge coping following the slope, party-wall chimneys raised by eight brick courses, a simple lead flashing at the change of pitch with no gutter or fascia and, on a full front-and-rear mansard, dormers no more numerous than the windows below and aligned with them. It also says rear mansards are generally not considered appropriate on heritage assets and buildings in conservation areas, where small dormers or roof lights are preferred. Other boroughs set different angles and priorities, and where neighbours already have mansards the council usually expects yours to match theirs, so we read the borough's guide and the earlier consents on your street before drawing.
Raising the party walls: a separate legal step
A mansard on a terrace or semi nearly always builds up the party walls, because the new side walls of the room sit on them. Under the Party Wall etc. Act 1996, raising a party structure is one of the works listed in section 2, and section 3 requires a party structure notice to be served at least two months before the work begins. Cutting into the wall for steel bearings is listed in the same section and usually goes on the same notice.
This is separate from planning permission and from building regulations, and it runs on its own clock, so we prepare the notices as soon as the design is fixed and the planning application is in. Our post on what party wall notices cost in London sets out the steps if a neighbour dissents, and our party wall notices and drawings service starts at £45 per adjoining owner.
What the application contains, and what it costs
A mansard application is a householder application. Our mansard loft conversion drawings start at £845 for a rear mansard, with the measured survey included: existing and proposed floor plans, all four elevations, a roof plan, a section through the new storey showing the pitch and head height, and a site location plan. Where the borough or a conservation area calls for it, we explain which supporting statement it wants before you commit. A front and rear mansard starts at £1,695, and a rear mansard with building regulations drawings starts at £1,845.
The council's householder application fee, structural calculations and party wall notices are separate costs, shown below. For the build itself, our mansard loft conversion cost guide explains the ranges and what moves them, and our post on how much structural calculations cost in London covers the engineer's part. Free redraws continue to approval, and if the head height under your ridge is too low for the borough's test, we tell you after the survey rather than draw a scheme that cannot work.
- Mansard planning drawings, survey included
- £845
- Council householder application fee
- £548
- Structural calculations
- £745
- Party wall notice, per adjoining owner
- £45
- Total
- £2,183
Sources
Every figure in this article, where it comes from and a link to the original.
- principal elevation: GPDO 2015 Sch 2 Pt 1 Class B.1(c): (source wording: "extend beyond the plane of any existing roof slope which forms the principal elevation of the dwellinghouse and fronts a highway"). www.legislation.gov.uk
- 40 cubic metres: GPDO 2015 Sch 2 Pt 1 Class B.1(d): added roof space limit (source wording: "40 cubic metres in the case of a terrace house" / "50 cubic metres in any other case"). www.legislation.gov.uk
- 0.2 metres: GPDO 2015 Sch 2 Pt 1 Class B.2(b)(i): eaves maintained or reinstated, and (source wording: "the edge of the enlargement closest to the eaves of the original roof is, so far as practicable, not less than 0.2 metres from the eaves"). www.legislation.gov.uk
- article 2(3) land: GPDO 2015 Sch 2 Pt 1 Class B.1(f): (source wording: "the dwellinghouse is on article 2(3) land"). www.legislation.gov.uk
- 1 July 1948: GPDO 2015 Sch 2 Pt 1 Class AA.1(c): additional storeys not permitted where (source wording: "the dwellinghouse was constructed before 1st July 1948 or after 28th October 2018"). www.legislation.gov.uk
- substantial weight: NPPF (August 2026) policy L2(1)(d): substantial weight to creating additional homes or floorspace using airspace above existing premises (source wording: "including, but not limited to, the addition of mansard roofs"); glossary: mansard suitable for a terrace of at least three buildings, at least two storeys, with a parapet. assets.publishing.service.gov.uk
- 70 degrees: Lambeth Design Guide SPD Part 4 (Aug 2023) para 4.102(2)-(3), rear mansards: (source wording: "Ensure the lower pitch of the rear mansard is 70 degrees and hung with slate") and terminate the 70 degree pitch at 2.3m where mansards in a row link up; para 4.101 not generally appropriate in conservation areas. www.lambeth.gov.uk
- 2.1 metres: Lambeth Design Guide SPD Part 4 (Aug 2023) para 4.102(1): (source wording: "There must be sufficient internal headroom (2.1m) below the existing ridge or the principle of a rear mansard will not be acceptable"). www.lambeth.gov.uk
- raise a party structure: Party Wall etc. Act 1996 s2(2)(a): building owner's right (source wording: "to underpin, thicken or raise a party structure, a party fence wall, or an external wall"). www.legislation.gov.uk
- two months: Party Wall etc. Act 1996 s3(2)(a): a party structure notice is served (source wording: "at least two months before the date on which the proposed work will begin"). www.legislation.gov.uk
Questions people ask
Can a mansard ever be permitted development?
Do I need planning permission for a rear mansard in a conservation area?
What pitch should a mansard be in London?
How much head height do I need for a mansard?
Does a mansard need a party wall notice?
Photographs: Rohan Gangopadhyay (Unsplash License); Himalay Patel (Pexels License); Chris Whippet (CC BY-SA 2.0); Clément Proust (Pexels License).
Mansard Loft Conversion Drawings: a fixed fee from £845
Read more on the mansard loft conversion drawings service page, or send your address below and we reply within one working day.
Mansard Loft Conversion Drawings
from £845
£645 drawings + £200 survey (£845 total)
Every price is the full fixed fee you pay. No VAT is charged. Council application fees are paid to the council separately.
What you get
- Measured survey of the roof, loft and floor below
- Class B, Class AA and conservation area check before design
- Head height test against your borough's mansard guidance
- Existing and proposed plans, elevations, roof plan and section
- Pitch, materials and dormers matched to the street's earlier mansards
- Party wall notices prepared once the design is fixed