Permitted development for upward extension of a house

Adding a storey to a house without a full planning application is possible under Class AA of Part 1 of Schedule 2 to the General Permitted Development Order 2015, but the qualifying conditions are narrow and the process is not the light touch one people expect. Class AA upward extension of a house allows up to two additional storeys on a house of two or more storeys, or one additional storey on a single storey house, subject to age, height and location limits, and it always requires a prior approval application to the council before work starts.

The right was introduced in 2020. It has been used far less than the government expected, mostly because the conditions rule out the streets where people most want to use it.

What the class permits

The permitted development is the enlargement of a dwellinghouse by construction of additional storeys immediately above the topmost storey, on the principal part of the house, together with the engineering operations needed to strengthen existing walls and foundations within the curtilage.

It applies to detached, semi-detached and terraced houses. It does not apply to flats, to buildings in other uses, or to a house that has already been extended upwards under this right.

Age and construction conditions

The house must have been built after 1 July 1948 and before 28 October 2018. That single condition removes most Victorian and Edwardian terraces, every interwar semi built before the war, and anything completed in the last few years.

The work must also be carried out so the new storeys sit above the principal part of the house, with a roof pitched or flat to match, and the engineering operations must be limited to strengthening existing walls and foundations. If the structure will not take the load without new foundations beyond that, the right does not cover the work.

Height limits and the neighbouring roofline

Three height controls apply at once, and the tightest one governs.

1. The highest part of the roof of the extended house must not exceed 18 metres.
2. The extended house must not exceed the height of the highest part of the roof of the existing house by more than 3.5 metres.
3. Where the house is attached to one or more other houses, the extended house must not be more than 3.5 metres higher than the highest part of the roof of the building it is attached to.

The floor to ceiling height of each new storey is limited to 3 metres, or the floor to ceiling height of any storey in the existing house, whichever is lower. On a mid-terrace with neighbours either side, the third control is the one that bites, and it is measured against the roof of the attached building rather than against your own.

At 18 metres a separate regime also comes into play, because buildings at or above that height engage the higher-risk building rules under the Building Safety Act. That threshold is very unlikely to be reached on a house, but the interaction matters where the site sits in a taller context. We look at those stages in the Building Safety Act gateways explained for applicants.

Where the right does not apply at all

Class AA is excluded on article 2(3) land, which covers conservation areas, National Landscapes, National Parks, the Broads and World Heritage Sites. It is also excluded on land that is or contains a listed building or a scheduled monument, on a site of special scientific interest, and on safeguarded land.

Article 4 directions remove it in places where the council has decided upward extension would harm local character. Check the designations for the property before spending money on drawings, and check whether an article 4 direction applies to the street rather than to the wider area.

Prior approval and what the council can consider

The application is for prior approval, made to the local planning authority with plans, a written description and the fee set in the current fee regulations. The council cannot refuse it on general planning grounds. It can only consider the matters the order lists:

• Impact on the amenity of adjoining premises, including overlooking, privacy and loss of light.
• External appearance, including the design and architectural features of the principal elevation and of any side elevation fronting a highway.
• Air traffic and defence asset impacts.
• The impact on any protected view identified in the Directions relating to protected vistas.

Loss of light is the one that decides most cases on terraced streets. A two storey addition on a narrow plot with neighbouring rear windows close to the boundary is hard to defend, and the council is entitled to refuse on that ground alone.

Prior approval also carries procedural conditions: the development must be completed within three years of the date of approval, and the developer must notify the authority in writing when it is finished. Some conditions require a report on the management of construction, covering hours, parking and the route for deliveries.

Design, materials and windows

The materials must be of a similar appearance to the existing house. That is a condition of the right rather than a matter of taste, and getting it wrong makes the development unlawful rather than merely unattractive.

Windows are the practical design constraint. Because overlooking is one of the prior approval matters, side elevation glazing is usually the first thing an officer asks to be removed or obscured. Plan the internal layout so the new storey works with rooflights and front and rear glazing, rather than designing around a side window you will lose.

When a planning application is the better route

Plenty of upward extensions fail one condition by a small margin: the house was built in 1946, the street is in a conservation area, or the neighbour's roof is 4 metres lower. A householder or full planning application then becomes the route, and it has an advantage the permitted development route does not. The council can grant something that does not match the existing materials, or that steps back from the boundary, because it is judging the proposal on its merits rather than against a checklist.

A planning application also lets you combine the upward extension with a rear extension and internal alterations in one permission. Where design is being argued, a short Design and Access Statement explaining the roofline, the materials and the relationship to neighbouring properties does more work than another elevation drawing.

The only formal way to confirm that works are lawful under permitted development is a Lawful Development Certificate. A prior approval decision confirms the matters the council assessed, not that every limit in the class has been met, so where the position is finely balanced, apply for a certificate as well.

Reform to permitted development rights is regular and the classes are amended often. Follow changes on the planning law updates timeline and verify the current text of the order on legislation.gov.uk before relying on any limit quoted above.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. You can also browse the report types a householder application may need. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.