Can you add an extra storey to your house without planning permission?

The limits

Rooms in the roof and storeys below ground do not count as storeys, and a new storey can replace rooms in the roof (AA.4(2)).

Development is not permitted by Class AA if ... the dwellinghouse is not detached and following the development the height of the highest part of its roof would exceed by more than 3.5 metres (i) in the case of a semi-detached house, the height of the highest part of the roof of the building with which it shares a party wall ... or (ii) in the case of a terrace house, the height of the highest part of the roof of every other building in the row in which it is situated

A common misreading

AA.1(f) limits the total rise of the roof: 7 metres on a house of more than one storey, however many storeys are added. The Planning Portal's summary says each added storey cannot add more than 3.5 metres; the Order sets no figure per storey. On a semi-detached or terraced house, AA.1(g) caps the roof at 3.5 metres above the attached neighbours' roofs, which in practice allows one storey, not two, unless they have already built up.

Prior approval

The council must approve four things (AA.2(3)(a)):

• the impact on adjoining premises, including overlooking, privacy and loss of light;
• the external appearance of the house, including the principal elevation and any side elevation facing a road;
• air traffic and defence assets;
• protected views identified in the Secretary of State's 2012 protected vistas directions.

You also send a construction management report setting out working hours and how noise, dust, vibration and traffic will be controlled (AA.2(3)(b)), with scaled plans and existing and proposed elevations showing the windows (AA.3(2)). Adjoining owners and occupiers get at least 21 days to comment (AA.3(5)). The council must have regard to the National Planning Policy Framework so far as it is relevant to those matters (AA.3(12)).

There is no deemed approval. Work cannot begin until you have written prior approval (AA.3(13)), and the council has 8 weeks to decide unless you agree longer (article 7). The work must be completed within 3 years of approval, and you must notify the council in writing when it is (AA.2(3)(c) to (e)).

What this means for you

Start with the build date: a house built before 1 July 1948 or after 28 October 2018 cannot use Class AA. Then check the designations and the planning history, and run the permitted development checker. Once Class AA has been used, loft conversions under Class B are no longer permitted development (B.1(h)). Building Regulations approval is needed for the new storeys.

Getting certainty

The prior approval application is itself the council's check against the limits: it can refuse if the scheme breaks one, or if you have not shown that it complies (AA.3(3)). Keep the approval and your completion notice with the deeds. For other work on the house that relies on permitted development, a lawful development certificate gives written confirmation.