A full planning application is the alternative where the rights do not apply. Planning permission does not establish that the existing structure can safely carry the proposed addition. [1][2][3]
Do not confuse the different Class AA routes
Part 20 Class AA concerns new dwellings above qualifying detached commercial or mixed-use buildings. Class AB concerns qualifying terrace buildings. These are different from Part 1 Class AA, which deals with enlarging an existing house by adding storeys. The shared letters do not make the eligibility rules interchangeable. [4]
The commercial rooftop rights also do not create a general entitlement to add office floors, enlarge every Class E building or put new homes above any shop. They concern the specific developments and existing buildings described in the relevant statutory class. [4][1]
Establish eligibility before drawing the extra floors
A proper feasibility check needs the building's age and use history, existing number of storeys, configuration, dimensions and surrounding constraints. The legislation distinguishes detached and terrace buildings and includes limits and exclusions relating to the existing building and proposed addition. [4]
Provide this information as an eligibility schedule rather than assuming that a flat roof is enough. Check the current legislation and address-specific restrictions before relying on the route. Where eligibility cannot be established, assess the full planning option instead of presenting permitted development as certain. [2]
Prior approval still examines the design
The commercial rooftop provisions include matters such as appearance, transport, flooding, contamination, natural light, neighbours' amenity and the effects of commercial activity on the new residents. Existing businesses and the introduction of residential use need to be considered together. [4]
A useful early design study should therefore show the relationship with adjoining windows, the retained commercial premises and the proposed access. It should not consist only of a diagram of the additional volume. Natural light, privacy and the impact on surrounding occupiers are part of the route's assessment, not matters automatically left until construction. [4]
Structure and fire access can determine feasibility
Assess the existing building's capacity, the structural alterations and the means of escape for both existing and new occupiers. The original rooftop provisions expressly address safe access and escape, while building regulations remain a separate control over the construction. [4][3]
Check whether the resulting building falls within the higher-risk-building regime. Mixed commercial and residential use does not by itself exclude that regime. Where it applies, the Building Safety Regulator is the relevant building-control authority; the ordinary choice between council building control and a registered building control approver is not available for that work. [3]
A sensible order of work
Start with the measured building and use-history evidence, then test the legal route, structural feasibility, residential layout and fire/access strategy together. Only after those checks should the proposal be developed into the relevant application package. This avoids treating a planning route as evidence that the building or proposed layout is technically suitable. [4][2][3]