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

01Harrow, London

Mansard roof drawings in Harrow

Harrow's Residential Design Guide asks for subordinate roof extensions, so a mansard here is argued as a design case, not assumed. We survey the roof, test it against the guide and draw it from £845 including VAT, with the roofline shown against both neighbours.

From £845 (VAT included)

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    02The brief

    What the Residential Design Guide says about roofs

    The council's guide (Residential Design Guide SPD) says dormers must stay subordinate: never above the ridge and never wrapping a hip. For semi-detached and terraced houses its line reads: hip-to-gable + rear dormer preferred. The rules we have recorded from the guide cover dormers and set-ins, and none names a mansard, so we treat a mansard as an exception to be justified drawing by drawing.

    The same section sets minimum set-ins from the party wall, the gable end and the eaves, a clear gap from any valley, and spacing between dormers. The table lists each wording with its page reference, and our drawings dimension every one of them.

    Brick cottage with a tiled roof, chimneys and a front dormer window
    Upper floors of a large yellow-brick Victorian house in London with chimney pots
    Photograph: Lorenc Memaga

    03The record

    Local figures for Harrow

    Figures with their unit, period and source
    MeasureValuePeriodSource
    Rear roof extension/dormer set in from party boundary and gable end, set back from eaves along slope500 mm party wall, 1000 mm gable, 1000 mm eaves mmNo periodharrow.gov.uk, Harrow Residential Design Guide SPD (adopted 15 December 2010), para 6.70, p.49
    Dormer clear of valley with return roof; top set down from ridge where possible500 mm from valley; 500 mm below ridge recommended mmNo periodharrow.gov.uk, Harrow Residential Design Guide SPD (adopted 15 December 2010), para 6.71, p.49
    Dormers subordinate, never above ridge or wrapping hips; hip-to-gable plus rear dormer preferred on semis/terraceship-to-gable + rear dormer preferredNo periodharrow.gov.uk, Harrow Residential Design Guide SPD (adopted 15 December 2010), paras 6.68-6.69, p.49
    Multiple rear dormers spaced apart; 500 mm from party wall, 1000 mm from hip or verge500 mm between dormers mmNo periodharrow.gov.uk, Harrow Residential Design Guide SPD (adopted 15 December 2010), para 6.72, p.50
    Class B, condition B.2(b)(i)(aa)Class B requires the original eaves to be maintained or reinstated, which a parapet-fronted mansard normally removesNo periodlegislation.gov.uk
    Class AA, para AA.1(b)Class AA is not available on article 2(3) landNo periodlegislation.gov.uk
    Householder: alterations/extensions to a single dwellinghouse, from 1 Apr 2026548 GBPNo periodribblevalley.gov.uk
    Listed building entries295 list entries (Historic England, within the LPA boundary)No periodplanning.data.gov.uk
    Householder applications granted81.7 %1 July 2025 - 30 June 2026assets.publishing.service.gov.uk
    Grade II listed entries265 list entriesNo periodplanning.data.gov.uk
    Approved Document B Vol 1, para 2.21New 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)No periodgov.uk
    Approved Document K 2013, para 1.13, Diagram 1.4Loft conversions where 2.0 m cannot be achieved: 1.9 m at the centre of the stair, 1.8 m at the side (half width)No periodgov.uk
    Party Wall etc. Act 1996 s3(2)(a)Party structure notice at least 2 months before workNo periodlegislation.gov.uk

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

    Harrow6 neighbouring councils
    Councils bordering Harrow

    04The streets

    Householder applications by street in Harrow

    Streets in Harrow with the most householder applications, and the share approved
    StreetApplicationsApproved
    Headstone Lane3179%
    Norman Crescent2878%
    Albury Drive2675%
    Suffolk Road2576%
    Howberry Road2496%
    Evelyn Drive2284%
    Moss Lane2263%
    Whitchurch Gardens2288%
    Cannonbury Avenue2190%
    Kings Road2153%
    Sylvia Avenue2195%
    Cannon Lane2084%
    Alicia Gardens1988%
    South View Road1961%
    Christchurch Avenue1860%

    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, Harrow: set-back and parapet: 500 mm party wall, 1000 mm gable, 1000 mm eaves mm. B, Permitted development and the eaves: Class B requires the original eaves to be maintained or reinstated, which a parapet-fronted mansard normally removes. C, Additional storeys under permitted development: Class AA additional storeys exclude houses built before 1 Jul 1948 or after 28 Oct 2018. D, 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)ABCD

    A mansard roof in section

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

    A Harrow: set-back and parapet
    500 mm party wall, 1000 mm gable, 1000 mm eaves mm
    B Permitted development and the eaves
    Class B requires the original eaves to be maintained or reinstated, which a parapet-fronted mansard normally removes
    C Additional storeys under permitted development
    Class AA additional storeys exclude houses built before 1 Jul 1948 or after 28 Oct 2018
    D 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

    Why the eaves decide the route

    Class B requires the original eaves to be maintained or reinstated, which a parapet-fronted mansard normally removes. Class AA does not rescue it. Class AA additional storeys exclude houses built before 1 Jul 1948 or after 28 Oct 2018. Class AA is not available on article 2(3) land.

    So a Harrow mansard goes to the council as a householder application. The council keeps £548 of the cost, and our fee comes on top of that.

    A listed roof is historic fabric

    Harrow has 295 list entries, 265 of them at Grade II. If yours is one, a roof alteration needs listed building consent beside planning permission, and the existing roof is read as part of what is listed. Listed building consent drawings in Harrow sets out how the council weighs it.

    The council's own figures for householder decisions, with 81.7% granted in the latest year, cover every kind of scheme. Use them as background and not as a forecast for a roof.

    The new storey: fire, headroom and the neighbours

    Fire rules reach the new floor. 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). Headroom on the stair is tested separately: Loft conversions where 2.0 m cannot be achieved: 1.9 m at the centre of the stair, 1.8 m at the side (half width).

    Shared roofs bring the party wall procedure: Party structure notice at least 2 months before work. A basement under the same house is a separate application, set out under basement conversion drawings in Harrow. The mansard roof drawings hub explains the London-wide route.

    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

    Rear view of brick terraced houses in London with slate roofs and chimney stacks along the shared rooflineParty wall services
    Photograph: Ben Hadfield
    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

    Can I build a mansard on a semi-detached house in Harrow?

    It is possible but it is not the route the council's guide prefers: for semis and terraces its line reads: hip-to-gable + rear dormer preferred. We test the roof against the guide in the free constraints check before quoting.

    Does Harrow allow dormers above the ridge?

    No. The guide says dormers stay subordinate and never rise above the ridge or wrap around a hip, and it recommends setting the top below the ridge.

    Which does Harrow prefer, a mansard or a hip-to-gable with a rear dormer?

    The guide recommends the second on semis and terraces, in the words hip-to-gable + rear dormer preferred. A mansard is possible but has to be justified drawing by drawing, so we compare both on the survey before you choose.

    Is a mansard in a conservation area harder?

    The upward-extension class is not available on that land, so a householder planning application is the route. Whether a conservation area covers your street is something we confirm before you commit.

    Converted loft bedroom with roof windows and a Juliet balcony

    81.7%

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

    Photograph: Imad Clicks

    09Further reading

    Other services in Harrow

    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 Harrow