Class AD: upward extension of a house to create new flats

Class AD upward extension flats are permitted on a detached dwellinghouse built after 1 July 1948 and before 5 March 2018, up to two additional storeys, subject to prior approval by the local planning authority. The right sits in Part 20 of Schedule 2 to the General Permitted Development Order 2015, and it creates separate new dwellings rather than enlarging the existing house. That last point is what separates it from Class AA in Part 1, which adds storeys to the same dwelling for the same household.

Most houses fail the test at the first or second question, so work through them in order before drawing anything.

What Class AD permits

The right allows the construction of up to two additional storeys on a detached dwellinghouse of two or more storeys, or one additional storey where the existing house has a single storey, to create one or more new dwellings. It also permits the engineering operations reasonably necessary to strengthen existing walls and foundations, works for access to and egress from the new flats, and the fitting of storage, waste or other ancillary facilities.

What it does not permit is extension beyond the curtilage, a basement, a new building in the garden, or any works to a house that is already in flats. Materials must be of a similar appearance to those used in the existing building. The approach to engineering works mirrors the familiar test under other classes, and the reasoning in our note on what counts as reasonably necessary structural work under Class Q transfers across.

The construction date window

The existing building must have been constructed after 1 July 1948 and before 5 March 2018. Both dates do real work. A Victorian villa is out. A house completed in 2019 is out. An estate house from 1974 is in, as far as this test goes.

Proving the date is the applicant's job. The approved drawings and the decision notice for the original permission are the cleanest evidence, followed by the building control completion certificate, the first council tax banding, and historic mapping showing the plot undeveloped on a dated edition and built on the next. Where a house was substantially rebuilt, the date of that rebuilding is arguable and the council will take a view. Say what your evidence is in the application rather than asserting a date.

Height limits and the relationship to neighbours

The Order caps both the finished height and the increase. The extended building must not exceed 18 metres overall, the increase over the highest part of the roof of the existing house is limited to 3.5 metres for one additional storey and 7 metres for two, and the floor to ceiling height of each new storey is limited to 3 metres or the height of the existing storeys, whichever is lower. Check the current class text, because these figures have been amended since the right was introduced.

On a two storey suburban house, adding two storeys produces a building roughly twice the height of its neighbours. That is the real constraint. The height may be within the numerical limits and still fail the prior approval test on amenity, because the authority is required to consider the effect on the amenity of the existing building and neighbouring premises, including overlooking, privacy and loss of light.

Prior approval matters the authority may consider

Prior approval under Class AD is far broader than the siting and appearance test used for some other rights. The authority may consider:

• Transport and highways impacts, including parking and servicing.
• Air traffic and defence asset impacts.
• Contamination risks and flooding risks on the site.
• The external appearance of the building, including the design of the new storeys.
• The provision of adequate natural light in all habitable rooms of the new dwellings.
• The impact on the amenity of the existing building and of neighbouring premises, including overlooking, privacy and loss of light.
• Whether, because of the siting of the building, the development will impact on a protected view identified in the relevant Directions.

Natural light to habitable rooms is a stated matter rather than a general design aspiration, so a plan with an internal bedroom or a single small dormer serving a deep room invites refusal. A Daylight and Sunlight Assessment covering both the new rooms and the effect on neighbouring windows is the document that answers two of the matters at once.

Where the right is simply unavailable

Class AD does not apply on article 2(3) land, which rules out conservation areas, National Landscapes, National Parks, the Broads and World Heritage Sites. It is unavailable for listed buildings and buildings within their curtilage, scheduled monuments, sites of special scientific interest, safety hazard areas and military explosives storage areas. Aerodrome safeguarding and defence considerations appear again as prior approval matters even where the right exists.

An Article 4 direction can remove the right for a defined area, and a number of authorities have made directions aimed at upward extension rights. Check the council's own register before you proceed, because the direction will not show on national constraint datasets.

Practical constraints that defeat most schemes

The legal tests are only half the problem. The ones that kill schemes in practice are structural, financial and procedural.

1. Foundations and walls designed for two storeys rarely take two more without underpinning, and the right permits only strengthening that is reasonably necessary, so a scheme that effectively rebuilds the house is outside the class.
2. The new flats need independent access, which on a detached house usually means an external stair or an internal core carved out of the existing accommodation, reducing what remains.
3. New dwellings are chargeable development for the Community Infrastructure Levy where a charging schedule is in force, and the levy falls due when development starts.
4. Each new flat is a separate planning unit with its own refuse, cycle storage, parking and amenity expectations, and the thinking in our piece on garden offices and the separate planning unit problem is the same logic applied to a different right.
5. Construction on an occupied house is slow and expensive, and Part 20 requires a report setting out the proposed hours of operation and how construction impacts will be mitigated.

Sound insulation between the new flats and the rooms below is a Building Regulations matter rather than a planning one, and on a 1960s timber upper floor it usually means a floating floor and an independent ceiling. That eats into the headroom the class allows, which is why schemes drawn to the 3 metre floor to ceiling limit often lose a storey once the acoustic build-up is set out properly. Price the structure and the acoustics before the architect finalises the sections.

Where the house fails the date window or the site is designated, a full planning application remains open and so does the commercial conversion route. Our explanation of Class MA vacancy and continuous use conditions covers the other prior approval path investors usually test next, and a Planning Statement is the document that carries the case on a full application.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.