Can I Add an Additional Storey Under Permitted Development?

Class AA has the strictest eligibility criteria of any permitted development right. Your property must meet all of the following conditions:

• The house must be detached. Semi-detached houses, terraced houses, and flats are not eligible.
• The house must have been originally built between 1 July 1948 and 28 October 2018. Houses built before 1 July 1948 or after 28 October 2018 do not qualify.
• The house must currently have no more than 2 storeys (excluding any basement that is not a storey).
• The house must not be a listed building.
• The house must not be in a conservation area, national park, AONB, World Heritage Site, or the Broads.
• There must not be an Article 4 direction in force that removes Class AA rights.
• The house must not have been created by a permitted development change of use (such as an office-to-residential conversion under Class O or similar).

These criteria immediately rule out the majority of houses in England. Class AA is primarily designed for detached post-war houses on larger plots.

Understanding what counts as a storey is important because Class AA limits are based on the number of existing storeys:

• A basement that is entirely below ground level is generally not counted as a storey. However, a semi-basement with windows above ground level may be counted.
• A loft that has been converted into habitable space (with dormers, rooflights, and a permanent staircase) is generally counted as a storey. An unconverted loft used only for storage is not.
• Mezzanines may or may not count depending on their size relative to the main floor area.

If you are unsure how many storeys your house has, your council’s planning department can advise. Getting this wrong could invalidate your entire Class AA application.

The number of storeys you can add and the maximum height increase depend on the existing house:

• A 1-storey house (bungalow) can add up to 2 additional storeys, with a maximum height increase of 7 metres above the existing height.
• A 2-storey house can add 1 additional storey, with a maximum height increase of 3.5 metres above the existing height.

The new storey(s) must be constructed using materials that are similar in appearance to the existing house. The engineering of the additional storey must also be designed to provide adequate support for the new construction — which is where the practical challenges really begin (see below).

Unlike most permitted development, Class AA always requires prior approval before work can begin. This is a formal application to your local planning authority, not a simple notification.

The process works as follows:

1. Submit a prior approval application to your council. The application must include detailed plans showing the external appearance of the proposed additional storey, including materials, windows, roof form, and relationship to the existing house.
2. The council will consult your neighbours. Adjoining owners are formally notified and given an opportunity to comment.
3. The council has 56 days from the date of your application to make a decision. They assess:
   • The external appearance of the development
   • The impact on the amenity of neighbours (overlooking, privacy, loss of light)
   • Air traffic and defence considerations (if relevant)
   • Whether the development would be safe given the local flood risk level
4. If the council approves, you receive a prior approval decision notice and can proceed.
5. If the council does not respond within 56 days, prior approval is deemed to be granted.

The current fee for a Class AA prior approval application is £120 per dwellinghouse, which is significantly less than a full planning application.

The prior approval assessment under Class AA is limited to specific matters. The council cannot refuse based on:

• Whether they think the house "needs" an additional storey.
• The internal layout or room arrangement.
• General design or architectural preferences beyond external appearance.
• The principle of the development (it is established by the permitted development right).

However, the council’s assessment of "external appearance" gives them considerable discretion. An additional storey that looks out of character with the streetscene, uses inappropriate materials, or creates an overbearing presence on neighbours is likely to be refused.

Adding an additional storey is one of the most structurally complex building projects you can undertake. The existing house was designed to carry its current load, not an additional storey on top. Key engineering considerations include:

• Foundation capacity — The existing foundations may need to be underpinned or strengthened to carry the additional weight. This is expensive and disruptive.
• Wall loadings — The existing walls must be assessed for their ability to carry the load of the new storey. Older cavity walls may need additional structural support.
• Temporary removal of the roof — The existing roof is typically removed entirely before the new storey is built and a new roof constructed on top. This means the house is open to the elements during construction — weather protection and timing are critical.
• Staircase — A new staircase is needed to access the additional storey, which takes space from the existing floor plan.

You will need a structural engineer to assess the existing structure and design the additional storey. This is not a project for a standard builder without engineering input.

Adding an additional storey under Class AA is a substantial project. Typical costs include:

• Architectural and engineering design — £5,000–£15,000 depending on complexity.
• Prior approval application — £120 fee.
• Construction costs — highly variable, but typically £1,500–£2,500 per square metre for the new floor area. A 60m² additional storey might cost £90,000–£150,000 in construction alone.
• Foundation work — if underpinning is needed, this can add £20,000–£50,000 or more.

The timeline from initial design to completion is typically 9–18 months. The prior approval application takes up to 56 days. Building regulations approval runs alongside design development. Construction itself can take 4–8 months depending on the scope and whether foundation work is required.

An additional storey will require full building regulations approval covering:

• Structural design — foundations, walls, floors, roof.
• Fire safety — a 3-storey house requires a protected staircase with fire doors, and possibly a sprinkler system depending on the design.
• Thermal performance — walls, roof, floor and windows must meet current insulation standards.
• Sound insulation — between the new and existing floors.
• Accessibility — depending on the design, Part M may apply to the new storey.

You should also consider whether a structural warranty is needed. Mortgage lenders may require a 10-year structural warranty for the additional storey, which typically means using a warranty provider such as NHBC, LABC Warranty, or Premier Guarantee.

Even though Class AA only applies to detached houses, there may still be party wall obligations if the additional storey involves:

• Excavation for foundation work within 3 or 6 metres of a neighbouring structure (Section 6).
• Work near or on a boundary line (Section 1).

If your detached house is on a narrow plot with neighbours close by, and the foundation works require excavation near their structures, you will need to serve party wall notices.

Because Class AA requires prior approval, the prior approval decision notice itself serves as your formal record that the development is lawful. However, if you want additional confirmation that the completed work matches the approved plans (for example, if minor changes were made during construction), you can apply for a Lawful Development Certificate after completion.