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

01Hillingdon, London

Mansard roof drawings in Hillingdon

Hillingdon's roof policy supports extensions at the rear only, below the ridge and no wider than a set share of the roof, so a full-width mansard is outside what it supports. We test a roof against those limits first, then draw what the policy allows from £845 including VAT.

From £845 (VAT included)

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02The brief

Rear only, below the ridge

The Part 2 Development Management Policies limit a roof extension to the rear and require it to be subservient, below the ridge and no wider than 2/3 of roof width. A mansard that lifts the ridge or spans the whole roof sits outside it.

In practice a scheme here is a carefully proportioned rear roof extension. Our drawings show its width against the original roof, its height against the ridge and its edges against the eaves and valleys.

Upper floors of a large yellow-brick Victorian house in London with chimney pots

Converted loft bedroom with roof windows and a Juliet balcony — Photograph: Imad Clicks

03The record

Local figures for Hillingdon

Figures with their unit, period and source
Measure	Value	Period	Source
Roof extensions on rear only, subservient, no more than two-thirds of average original roof width, below ridge.	2/3 of roof width	No period	hillingdon.gov.uk, Hillingdon Local Plan Part 2 Development Management Policies 2020, Policy DMHD 1 E(i)
Hip-to-gable conversions and raising the main ridge are generally not supported.	no hip-to-gable; no ridge raise	No period	hillingdon.gov.uk, Hillingdon Local Plan Part 2 Development Management Policies 2020, Policy DMHD 1 E(ii)-(iii)
In conservation areas and on listed buildings, roof extensions should be traditional rear dormers kept away from ridge, eaves and valleys.	traditional rear dormers only	No period	hillingdon.gov.uk, Hillingdon Local Plan Part 2 Development Management Policies 2020, Policy DMHD 1 E(v)
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 removes	No period	legislation.gov.uk
Class AA, para AA.1(c)	Class AA additional storeys exclude houses built before 1 Jul 1948 or after 28 Oct 2018	No period	legislation.gov.uk
Class AA, para AA.1(b)	Class AA is not available on article 2(3) land	No period	legislation.gov.uk
Householder: alterations/extensions to a single dwellinghouse, from 1 Apr 2026	548 GBP	No period	ribblevalley.gov.uk
Party Wall etc. Act 1996 s3(2)(a)	Party structure notice at least 2 months before work	No period	legislation.gov.uk
Listed building entries	436 list entries (Historic England, within the LPA boundary)	No period	planning.data.gov.uk

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

Hillingdon7 neighbouring councils

Councils bordering Hillingdon

Three Rivers
Buckinghamshire
Ealing
Hounslow
Harrow
Slough
Spelthorne

04The drawing

A mansard roof in section

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

A Permitted development and the eaves: Class B requires the original eaves to be maintained or reinstated, which a parapet-fronted mansard normally removes
B Additional storeys under permitted development: Class AA additional storeys exclude houses built before 1 Jul 1948 or after 28 Oct 2018
C 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.

05The detail

Hip-to-gable and ridge raising

Two moves that owners often assume are available are closed by the same policy: no hip-to-gable; no ridge raise. On a hipped semi, that removes the usual way of gaining floor area at the side, and the rear roof has to do the work.

We therefore measure the true roof plan before talking about rooms. The width share is taken from the original roof, so earlier alterations to it matter.

Older streets and listed houses

For conservation areas and listed buildings the policy narrows the answer further: traditional rear dormers only. A mansard has little room in that sentence, and the design effort goes into a traditional rear dormer set well clear of ridge, eaves and valleys.

The borough holds 436 list entries, so check before you assume your house is not one. A listed roof needs consent as well, with the council's tests under listed building consent drawings in Hillingdon.

Where permitted development runs out

Permitted development is the quicker route and it rarely fits. Class B requires the original eaves to be maintained or reinstated, which a parapet-fronted mansard normally removes. For storeys added under Class AA, Class AA additional storeys exclude houses built before 1 Jul 1948 or after 28 Oct 2018. In addition, Class AA is not available on article 2(3) land.

That leaves a householder application, with a council fee of £548. We design to the rear-only limits so the application asks for what the policy can grant.

Neighbours and the next application

Work to a shared roof needs the neighbours' notice: Party structure notice at least 2 months before work. The drawing set also covers the fire and thermal rules for the new floor.

A lower floor under the same house is a separate application: see basement conversion drawings in Hillingdon. For how a neighbouring borough words its roof guidance, compare mansard roof drawings in Harrow, or read the mansard roof drawings hub.

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.

06The papers

What the drawings rest on

Brick cottage with a tiled roof, chimneys and a front dormer window — Photograph: Tanya Barrow

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.

Does Hillingdon allow hip-to-gable loft conversions?

Generally not. The council's policy, in short, is no hip-to-gable; no ridge raise.

How wide can a roof extension be in Hillingdon?

No wider than 2/3 of roof width, and at the rear only. We measure your roof before quoting so you know what that means for your house.

What changes in a conservation area?

The policy for conservation areas and listed buildings is: traditional rear dormers only.

Can I build one bigger dormer instead of a mansard?

Only inside the same limits. A dormer is still a roof extension at the rear, below the ridge and within the width share, so size is set by the original roof and not by how it is built.

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

08Further reading

Mansard roof drawings

Other services in Hillingdon

Mansard roof drawings nearby

09Sources

Sources and dates

hillingdon.gov.uk, retrieved 17 October 2026
legislation.gov.uk, retrieved 15 October 2026
ribblevalley.gov.uk, retrieved 17 October 2026
legislation.gov.uk, retrieved 15 October 2026
planning.data.gov.uk, retrieved 17 October 2026
gov.uk, retrieved 15 October 2026

Licences

Contains public sector information licensed under the Open Government Licence v3.0.

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

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