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Slate-clad roof extension on a yellow-brick London terraced house
Photograph: Daniel Sikpi

01Westminster, London

Mansard roof drawings in Westminster

Westminster states the geometry of an acceptable mansard before you draw it: front and rear slopes no steeper than 70 degrees, an upper slope not normally steeper than 30 degrees, and a parapet with raised party walls. We draw to those limits, show the roof against its terrace, and quote from £845 including VAT.

From £845 (VAT included)

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    02The brief

    The Roofs SPG, rule by rule

    The council's Roofs: a guide to alterations and extensions on domestic buildings SPG sets the numbers. The principal front and rear slope should be pitched at no more than 70 degrees, and the upper slope should not normally exceed 30 degrees. The mansard sits behind a parapet gutter at front and rear, with party walls raised along the slopes to a minimum upstand of 375 mm.

    Our sections label each of those dimensions, so an officer can check the roof against the guidance line by line.

    Converted loft bedroom with roof windows and a Juliet balcony
    Rear view of brick terraced houses in London with slate roofs and chimney stacks along the shared roofline
    Photograph: Ben Hadfield

    03The record

    Local figures for Westminster

    Figures with their unit, period and source
    MeasureValuePeriodSource
    On a mansard roof extension the principal (front and rear) slope should be pitched no greater than 70 degrees.70 degreesNo periodwestminster.gov.uk, Roofs SPG, PDF p.11, Designing a mansard roof, rule 1
    The upper slope of a mansard should not normally be greater than 30 degrees.30 degreesNo periodwestminster.gov.uk, Roofs SPG, PDF p.11, Designing a mansard roof, rule 2
    The mansard should be set back behind a parapet gutter at front and rear, with raised party walls following the roof slopes at the minimum upstand of 375 mm.375 mmNo periodwestminster.gov.uk, Roofs SPG, PDF p.11, Designing a mansard roof, rules 5-7
    Supplementary planning documentRoofs: a guide to alterations and extensions on domestic buildings SPGNo periodwestminster.gov.uk
    Conservation areas56 named conservation areas published by the councilNo periodplanning.data.gov.uk
    Article 4 directions10 Article 4 directions published by the councilNo periodplanning.data.gov.uk
    Listed building entries4,015 list entries (Historic England, within the LPA boundary)No periodplanning.data.gov.uk
    Householder decisions366 decisions1 July 2025 - 30 June 2026assets.publishing.service.gov.uk
    Householder applications granted82 %1 July 2025 - 30 June 2026assets.publishing.service.gov.uk
    Class AA, para AA.1(b)Class AA is not available on article 2(3) landNo periodlegislation.gov.uk
    Party Wall etc. Act 1996 s3(2)(a)Party structure notice at least 2 months before workNo periodlegislation.gov.uk
    Householder: alterations/extensions to a single dwellinghouse, from 1 Apr 2026548 GBPNo periodribblevalley.gov.uk

    A blank value means the figure has not been published, not that there are none. Check your own address with the council.

    Westminster7 neighbouring councils
    Councils bordering Westminster

    04The streets

    Householder applications by street in Westminster

    Streets in Westminster with the most householder applications, and the share approved
    StreetApplicationsApproved
    Randolph Avenue1679%
    Ashworth Road1385%
    Clifton Hill1170%
    Ponsonby Place10100%
    Lancaster Mews938%
    Westbourne Park Road988%
    Carlton Hill886%
    Kilburn Lane886%
    Sutherland Place863%
    Wilton Row8100%
    Caroline Close733%
    Chester Row7100%
    Greenberry Street757%
    Hereford Road760%
    Hyde Park Gardens Mews775%

    Counts of householder applications by street; no individual applications are shown. Streets with fewer than five applications are left out.

    GLA Planning London Datahub, householder applications

    05The drawing

    A mansard roof in sectionA, Westminster: lower slope: 70 degrees. B, Westminster: set-back and parapet: 375 mm. C, Permitted development and the eaves: Class B requires the original eaves to be maintained or reinstated, which a parapet-fronted mansard normally removes. D, Additional storeys under permitted development: Class AA additional storeys exclude houses built before 1 Jul 1948 or after 28 Oct 2018. E, Fire protection to the stair: New storey above 4.5 m by loft conversion: protect the full escape route with fire-resisting doors (min E 20) and partitions (min REI 30)ABCDE

    A mansard roof in section

    A London terrace with a mansard behind the parapet. The lettered marks show the limits the legend gives.

    A Westminster: lower slope
    70 degrees
    B Westminster: set-back and parapet
    375 mm
    C Permitted development and the eaves
    Class B requires the original eaves to be maintained or reinstated, which a parapet-fronted mansard normally removes
    D Additional storeys under permitted development
    Class AA additional storeys exclude houses built before 1 Jul 1948 or after 28 Oct 2018
    E Fire protection to the stair
    New storey above 4.5 m by loft conversion: protect the full escape route with fire-resisting doors (min E 20) and partitions (min REI 30)
    The mansard in three dimensions3D
    Scroll the page: the model turns with it.

    06The detail

    When the terrace decides

    The guidance also names the cases where a roof extension is not normally acceptable: complete terraces with a largely unimpaired roofline, buildings significantly higher than their neighbours, and an unbroken run of butterfly roofs. We survey the whole terrace roofline before we propose anything.

    Mews buildings follow a separate guide, Mews: a guide to alterations SPG. It expects an extension to take the form of a mansard, flat-topped or double-pitched, and it identifies some mews where roof extensions will not normally be permitted.

    Conservation areas, Article 4 and listed houses

    Westminster publishes 56 conservation areas and 10 Article 4 directions, and Historic England records 4,015 list entries here. Class AA is not available on article 2(3) land, so a conservation area means a householder planning application. A listed house adds consent, covered by listed building consent drawings in Westminster.

    How householder cases have gone in the borough

    Householder cases here run to 366 decisions across the twelve months of the statistics, with 82% granted and 79% decided in time. They are history, not a forecast.

    Whichever street you are on, the Party Wall etc. Act 1996 sets the notice: Party structure notice at least 2 months before work. The period streets of Marylebone and Knightsbridge each have a guide to their conservation areas, and the mansard roof hub compares boroughs.

    Planning sheet for a single-storey rear extension: proposed ground floor plan and rear elevation, with the Property Architecture title block
    Sample drawing. Proposed plan and rear elevation from a planning application set, sheet 03 at 1:50.

    07The papers

    What the drawings rest on

    Upper floors of a large yellow-brick Victorian house in London with chimney potsFlat conversions & change of use
    Photograph: Lorenc Memaga
    Loft floor plan marked up in an independent review, with three critical issues clouded and explained
    Sample drawing. A loft plan with the reviewer’s mark-up: stair pitch, dormer cheeks and roof ventilation.

    08Correspondence

    Questions

    How steep can a mansard slope be in Westminster?

    The Roofs SPG says the front and rear slope should be no steeper than 70 degrees, and the upper slope should not normally exceed 30 degrees.

    Does Westminster allow a mansard on a mews house?

    It can, in the form the guide expects. The mews guide calls for a mansard, flat-topped or double-pitched, but names some mews where roof extensions will not normally be permitted. Send the address and we check it against that list.

    Can I build a mansard in a Westminster conservation area?

    Through a householder application, yes. Class AA is not available on article 2(3) land, so permitted development is not the route, and the terrace roofline is what the officer weighs.

    What does the council charge for the application?

    Westminster charges £548 for a householder application, collected when it is lodged. Our own fee starts at £845 including VAT.

    Brick cottage with a tiled roof, chimneys and a front dormer window

    82%

    Householder applications granted, Westminster, 1 July 2025 - 30 June 2026. Source: assets.publishing.service.gov.uk

    Photograph: Tanya Barrow

    09Further reading

    Other services in Westminster

    Mansard roof drawings nearby

    10Sources

    Sources and dates

    Licences

    Kensington street lined with brick townhouses and lit streetlamps beneath an orange dusk sky

    Send us the address of your house.

    We look up the conservation area, listing and Article 4 position, then reply with a fixed fee. A message before 3pm is answered the same working day.

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    Mansard roof drawings in Westminster