Industry News
Building upwards in London: what the call for a rooftop extension pattern book means for homeowners
London Assembly members want City Hall to publish a ready-made design code for adding storeys and mansard roofs to existing homes, so that homeowners who follow it can expect consent. The Mayor has been asked to respond by 3 November. Until then, the existing permitted development and planning rules still apply.
Adding a floor to an existing house is one of the oldest ways London has grown. Mansard roofs on Georgian and Victorian terraces are part of the character of streets across Islington, Camden and Kensington and Chelsea. City Hall is now being urged to make this kind of work easier to get approved. This article explains what has been proposed, how it fits with the draft London Small Site Design Code, and what the rules for homeowners are today. It is general information, not planning or legal advice.
What the London Assembly has asked for
On 22 September 2026 James Small-Edwards AM, Chair of the London Assembly's Planning and Regeneration Committee, wrote to the Mayor of London about upward extensions. The letter, published in the committee's papers for 1 October, follows a meeting on 8 July with industry professionals and London boroughs. The Architects' Journal reported it on 7 October.
The committee's main recommendation is that the Greater London Authority (GLA) should, within 12 months, develop a pattern book or design code for upward extensions. Boroughs could adopt it in full, adapt it or produce their own version, and designs should be subject to "good local consultation and support". The committee has asked the Mayor to respond by 3 November 2026.
Why the committee thinks it is needed
Guests at the July meeting gave estimates of how many new homes upward extensions could provide in London. These ranged from about 140,000 up to a theoretical, technically feasible upper limit of half a million. Guests also pointed out that extensions add bedrooms to existing homes as well as creating new ones. Even so, the letter says actual delivery "has been comparatively low".
The main reason given is uncertainty. Dr Charles Gillott of the engineering firm WSP, whose doctoral research examined upward extensions, described technical questions such as whether an engineer can find enough spare structural capacity in a building, and financial questions about whether the work will pay off. The letter adds that fire safety changes since Grenfell, which it calls necessary, have added to the technical and financial challenges.
Boroughs that already use design codes
The committee heard from two boroughs, Tower Hamlets and Haringey, that already give residents a framework for roof extensions. Michael Ritchie of Tower Hamlets told members that the borough's codes let residents know "that if they meet the conditions of the design code, they will get consent for their mansard roof extension".
Tower Hamlets has published mansard guidance for the Driffield Road and Medway conservation areas, adopted in 2017. Applications there are also expected to include improvements to the front of the house and a financial contribution to a streetscape enhancement fund. Its June 2025 guidelines for Tredegar Square are very specific. They call for a 70-degree lower slope and a 30-degree upper slope, a setback of at least 900mm from the front facade, natural Welsh slate and lead flashings, and retention of the V-shaped rear parapet. In Haringey, the South Tottenham house extensions guidance has set out three approved forms of roof extension since 2010 (revised in 2013). Planning permission is still usually needed.
How this links to the draft Small Site Design Code
The letter is closely tied to the draft London Small Site Design Code, which the GLA published for consultation on 11 September to accompany the draft London Plan published in July. The code covers sites under 0.25 hectares and supports draft London Plan Policy MBUL2. Under that policy, schemes that comply with the code should be treated by boroughs as acceptable in principle in design terms. It is intended as a baseline where a borough has no small-site code of its own, and boroughs may still issue local versions. According to the Architects' Journal, it sorts sites into 12 types under three headings (infill, redevelopment and reconfiguration). It then applies four character contexts (suburban, terraced, high street and flatted) and sets dimensional limits such as the number of storeys.
The consultation closes on 4 December 2026. The code is aimed mainly at small housing schemes rather than single-house alterations, and it is not the rooftop pattern book the committee wants. The committee sees it as a model for how a London-wide upward extensions code could work.
What the rules are today
None of this changes the law yet. A homeowner thinking about an extra storey or a mansard currently has two main routes.
Permitted development: Class AA
Since August 2020, Class AA of the General Permitted Development Order has allowed a house to be extended upwards without a full planning application, subject to prior approval from the council. In summary:
- A house of two or more storeys can add up to two storeys; a single-storey house can add one.
- The house must have been built between 1 July 1948 and 28 October 2018. Class AA is not available in conservation areas or on other article 2(3) land.
- The roof must not exceed 18 metres, or rise more than 7 metres above the existing roof (3.5 metres for a single-storey house). A semi-detached or terraced house must not end up more than 3.5 metres higher than its neighbours.
- Each new storey can be no more than 3 metres from floor to ceiling, and no higher than the existing storeys. It must sit on the main part of the house, with matching materials, the same roof pitch and no windows in the side walls.
- The council assesses the impact on neighbours, including overlooking, privacy and loss of light, along with the appearance of the front elevation and any protected views. It must give adjoining owners and occupiers at least 21 days to comment. You must also submit a construction management report, pay a fee and complete the work within three years.
Because the 1948 cut-off rules out most Georgian, Victorian and Edwardian houses, Class AA does not cover much of inner London's terraced housing. Many terraces are also in conservation areas. The Architects' Journal notes that the government consulted in 2024 on changing the 1948 requirement, and the proposal is still under consideration. For the related prior approval route for ground-floor extensions, see our guide to larger home extension prior approval.
Householder planning permission
Most mansards and extra storeys in London therefore still need a householder planning application. Your borough will judge it against its local plan, any conservation area guidance and the London Plan. This is where design codes matter: where a borough has adopted clear rules for roof extensions, as Tower Hamlets has in some conservation areas, you can design to them from the start. Our guide to dormer, hip-to-gable and mansard lofts explains the main roof forms. Our article on the national scheme of delegation covers how householder applications are decided.
Practical points before you plan an extra storey
- Check your borough's guidance. Look for conservation area appraisals, roof extension guidance or a design code that covers your street. Ask whether the borough plans to adopt the London-wide code once it is final.
- Get a structural assessment early. Adding weight to an older house may mean strengthening walls or foundations. That uncertainty was the main barrier identified by the committee.
- Plan for Building Regulations. A new storey affects fire escape routes, stairs, insulation and structure, and needs building control approval. See our guides to loft conversion regulations and building control charges.
- Talk to your neighbours. Raising a party wall or building on it usually involves the Party Wall Act, and neighbours can comment on a prior approval or planning application.
- Budget realistically. Our London extension and loft cost guide gives indicative 2026 ranges. A full extra storey is a bigger job than a typical loft conversion, so get quotes based on proper drawings.
Whether you live in Lambeth, Hammersmith and Fulham or Barnet, you can request quotes from independent local builders once your design is settled. Home Builders does not carry out building work: we ask advertisers to provide proof of insurance, and any contract is between you and the builder you choose.
Sources: London Assembly Planning and Regeneration Committee, 1 October 2026 papers, including the letter to the Mayor on upwards extensions dated 22 September 2026; Architects' Journal, 7 October 2026; GLA: London Small Site Design Code consultation; Architects' Journal, 23 September 2026; The Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2, Part 1, Class AA; Tower Hamlets Council: mansard roof proposals in Driffield Road and Medway conservation areas; Tower Hamlets: Tredegar Square design guidelines for mansard roof extensions, June 2025; Haringey Council: house extensions in South Tottenham SPD; Planning Portal. This article is a summary, not legal or planning advice; check the position for your property with your borough's planning service.
Frequently asked questions
What is an upward extensions pattern book?
It is a set of pre-agreed designs and rules for adding storeys or mansard roofs to existing buildings. If a proposal follows the pattern book, it should be easier to get approved. The London Assembly's Planning and Regeneration Committee has asked the GLA to produce one within 12 months, which boroughs could adopt, adapt or replace with their own.
Has anything changed for homeowners yet?
No. The committee's letter is a recommendation to the Mayor, who has been asked to respond by 3 November 2026. The draft London Small Site Design Code is out for consultation until 4 December 2026. The current permitted development and planning rules still apply.
Can I add a storey to my London house without planning permission?
Only if the house qualifies under Class AA permitted development. It must have been built between 1 July 1948 and 28 October 2018, must not be in a conservation area, and must meet height and design limits. Prior approval from the council is still required. Most older terraces need a planning application instead.
Does a mansard roof need planning permission in London?
Usually, yes. A mansard generally goes beyond householder permitted development limits, and many London terraces are in conservation areas. Check your borough's conservation area or roof extension guidance before you commission drawings.
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