This is an enlargement of the existing house
The relevant right is Schedule 2, Part 1, Class AA of the General Permitted Development Order. The full reference matters because different parts of the Order use similar class labels. This right enlarges a dwellinghouse; it is not a general permission to create new flats on top of a building. The completed house must remain in the specified residential use, with only ancillary other purposes. [1]
Check the building's eligibility first
The house must have been constructed between 1 July 1948 and 28 October 2018. The right is excluded where the existing house has already been enlarged by additional storeys above the original house, whether through this right or otherwise. Houses authorised only through specified permitted development conversion routes are also excluded. [1]
The right is unavailable on article 2(3) land, including conservation areas, or on a Site of Special Scientific Interest. Flats and buildings containing flats do not qualify for the householder right. Conditions on existing permissions and any relevant Article 4 direction must also be checked. [3][4][5]
Establish these points before spending time developing an extra-storey layout. Evidence of construction date, the original building and the permission history can determine whether Class AA is available at all.
The height and form limits work together
The finished house must not exceed 18 metres to the highest part of its roof. The increase above the existing roof is limited to 3.5 metres for a single-storey house or 7 metres for a house with more than one storey. Additional relative-height restrictions apply to attached houses: the proposal cannot simply ignore the adjoining building or the other buildings in a terrace. [1]
Each added storey must be no more than 3 metres internally from floor to ceiling, and no higher internally than any existing storey in the principal part. Where existing storey heights differ, do not select only the tallest as the comparison. [1][3]
For the number-of-storeys test, below-ground storeys and accommodation within the existing roof do not count. A basement or converted loft therefore does not automatically make a single-storey house eligible for two additional storeys. [1]
New storeys must be above the principal part of the house, not lower front, side or rear extensions. The principal roof pitch must remain the same, exterior materials must have a similar appearance, and the development must not introduce a window in a side-facing wall or roof slope. Visible external support structures and the scope of associated engineering works are also restricted. [3][1]
What prior approval assesses
The specified matters include:
- Effects on adjoining amenity, including overlooking, privacy and loss of light.
- External appearance, including the design of the principal elevation and highway-facing side elevations.
- Air-traffic and defence impacts, and the protected views identified in the legislation. [2][1]
A compliant height is therefore not a guarantee of approval. The application needs scaled information showing the existing and proposed building and enough evidence to address the relevant assessment matters. [3][1]
Approval, construction information and completion
Government guidance gives an 8-week determination period. Class AA expressly requires the applicant to receive written prior approval before starting. If the council does not decide within the applicable period, consider the non-determination appeal route rather than importing the larger rear-extension rule. [2][1]
Before beginning, the developer must also provide the council with the required construction-management report, covering proposed operating hours and mitigation of noise, dust, vibration and traffic effects on adjoining owners or occupiers. Development must be completed within 3 years of the prior-approval grant, and completion must be notified in writing as soon as reasonably practicable. This is a completion deadline, not simply a deadline to start. [1]
Building regulations, structural design and any Party Wall Act process remain separate requirements. Planning prior approval is not confirmation that the existing building can safely carry the proposed extra floors. [6][7]