Borough roof guidance depends on the building, street and heritage designation, not just the word “mansard”. Read the relevant adopted policy and local guide, and check national permitted-development rights separately. A guide supporting a particular roof form does not itself grant permission for that roof. [1][2][3]
The national policy context changed in August 2026
The NPPF published on 17 August 2026 uses policy L2 to address effective use of land. It gives substantial weight to the benefits of specified forms of intensification, including opportunities to use airspace and mansard roofs, while retaining relevant design, access and amenity considerations. This is decision-making policy, not a replacement permitted-development allowance. [4][5]
For an application, explain why the proposal is an appropriate use of the existing building and how it responds to its surroundings. General national support is not a reason to omit the local character assessment or the effect on neighbouring properties. [5]
Tower Hamlets: specific mansard guidance, not a borough-wide entitlement
Tower Hamlets publishes mansard guidance for Clinton Road, Driffield Road, Medway, Ropery Street and Tredegar Square conservation areas. It concerns suitable terrace forms, including qualifying parapet and London or butterfly roofs. It does not cover every house simply because it falls inside one of those areas. [1]
The council identifies exclusions including listed and locally listed buildings and unsuitable existing roof forms. The guidance also addresses the relationship between the mansard, dormers, chimneys and the host building, and links proposals to façade heritage improvements and a streetscape contribution. Confirm any streetscape contribution with the council before budgeting. [1]
A useful first step is therefore to establish that both the building and the conservation-area location fall within the guide's scope before preparing a compliant-looking roof elevation. [1]
Brent: compare the proposal with the street's roof character
Brent's Residential Extensions and Alterations SPD, adopted on 16 January 2025, provides a different local design framework. Its roof guidance links front dormers to whether they are already a common feature of the street and addresses the position and scale of dormers in relation to the original roof. [2]
For example, the guidance keeps dormers within the original ridge height. It also distinguishes front, side and rear roof changes rather than treating every dormer in the same way. These are borough design considerations for assessing proposals, not a new national permitted-development volume calculation. [2]
The practical comparison is not “Brent allows dormers and Tower Hamlets allows mansards”. Each document has its own scope and qualifications, and the appropriate response depends on the original building and street. [2][1]
Westminster: an older roof guide needs a current policy check
Westminster still hosts its 2004 guide to alterations and extensions on domestic roofs. It explains roof form and architectural context, but its historical policy references should not be copied into a current application as though they were the latest City Plan. [6]
Westminster's current policy list records a City Plan partial review adopted on 21 January 2026. Use that operative framework to establish the planning policy position, and identify what role the older roof guide can properly play in explaining the design. The date on an old technical illustration does not make it a current development-plan policy. [7][6]
Keep permission, guidance and evidence separate
National householder permitted-development guidance has its own roof-extension limitations and conditions, including restrictions relating to designated land and the type of property. A local guide cannot override those national conditions or restore a right removed for the property. [3]
A practical roof-design file should therefore separate the permission route from the local design response. Record the existing roof and neighbouring roofline, identify the applicable guide and designation, and explain how the proposed form meets the relevant tests. This is more useful than relying on an approval elsewhere without checking whether the same building type and policy framework apply. [2][1][5]