Owning roof space is not, by itself, planning permission to develop it. [1] [2]
Establish whether the national route is available
Government planning guidance describes a route for up to two additional storeys of self-contained homes above an existing, freestanding, purpose-built block of flats of at least three storeys. It is subject to detailed restrictions: the guidance includes an overall height limit of 30 metres and a maximum increase of seven metres. These headline dimensions are not the complete eligibility test. [1]
Check the building's history, configuration, existing lawful use, location and the precise works proposed against the current General Permitted Development Order. Designated areas and listed-building circumstances can exclude the route. A block that happens to have three floors is not necessarily an eligible purpose-built block. [1]
Prior approval is a substantive stage
The council considers specified matters rather than the full planning merits in the usual way. Government guidance identifies matters including appearance, natural light, neighbouring amenity, transport and highways, flooding and contamination. The proposal still needs drawings and supporting evidence addressing the matters relevant to it. [1]
For this upward-extension route, the guidance gives an eight-week determination period, but expressly states that there is no deemed approval merely because that time has passed. Where no decision arrives, the options include waiting or appealing against non-determination; starting on an assumed automatic consent is not the correct response. [1]
Test the whole building, not just the new floors
A rooftop layout must work with the building below. Establish a technical brief covering the existing structure, the proposed load path, escape arrangements, access and the services needed by the new homes. Building regulations approval is a separate requirement from prior approval. [2] [3]
Assess the height and use of the enlarged building against the higher-risk building criteria. For the design-and-construction regime, height of at least 18 metres or at least seven storeys can trigger the regime where the relevant use criteria are also met. Do not apply the height test in isolation or assume that the original building's classification settles the enlarged proposal. [4]
Recheck the staircase and energy position
Approved Document B's second-stair guidance took effect on 30 September 2026, with transitional arrangements. It uses the top-storey 18-metre threshold for blocks of flats, which is not the same wording as the higher-risk building test. Existing-building work and proposed additional floors need a coordinated fire strategy rather than a threshold-only sketch. [3] [5]
The Future Homes and Buildings Standards provisions commence on 24 March 2027 for relevant non-higher-risk work. The separate higher-risk building provisions commence on 24 September 2027, subject to their application-based transitional arrangements. Confirm the relevant regulatory route before setting the technical specification. [6]
Where the national planning route is unavailable, assess a full application against current national and local policies. The NPPF's support for sensitive use of airspace is relevant, but it does not remove the safeguards for character and living conditions. [7]
From 2 September 2029, Approved Document B removes BS 476 references for fire-resistance classification, subject to its published transitional arrangements. The applicable edition belongs in the enlarged building's fire strategy and construction specification. [8]