Class AA upward extension or a full application

Class AA upward extension prior approval is a narrow right with a wide reputation. Part 20 of Schedule 2 to the Town and Country Planning (General Permitted Development) (England) Order 2015 allows additional storeys to be built on certain existing buildings without a full planning application, subject to a prior approval procedure. The attraction is obvious. The conditions and exclusions are long enough that a sizeable proportion of schemes end up applying for planning permission anyway, and knowing which route applies before spending money on design is the point of this comparison.

Part 20 contains several classes rather than one. They differ by whether the existing building is detached or in a terrace, whether it is in residential or commercial and mixed use, and how many storeys may be added.

What Class AA permits above an existing block

The classes in Part 20 cover new dwellings created by building upwards. In outline, one class deals with detached blocks of flats, one with detached buildings in commercial or mixed use, one with terrace buildings in commercial or mixed use, one with terraced dwellinghouses and one with detached dwellinghouses.

The new units have to be dwellings, and in most of the classes they have to be flats. The right also covers the engineering operations reasonably necessary to construct the additional storeys, which means strengthening existing walls and foundations and providing or replacing water, drainage, electricity and gas services, plus works for safe access and egress and for storage and waste.

What the right does not cover is anything beyond the existing footprint. No extension sideways, no new building within the curtilage, no plant beyond what the Order allows.

Height caps and storey limits

The caps are where most schemes fail before they start.

A resulting height limit applies to the building as extended, and it differs between the classes: the limit is 18 m for the commercial, mixed use and dwellinghouse classes, and 30 m for the class dealing with detached blocks of flats. There is also a cap on how much the height may increase, which differs between the classes permitting one additional storey and those permitting two.

Each new storey is limited in internal floor to ceiling height, to 3 m or to the height of the existing storeys, whichever is lower. That last qualification is the one that catches conversions of older commercial buildings with generous floor to floor dimensions, because a building with 4 m existing storeys does not get 4 m new ones.

Measure the existing building accurately before testing any of this. Height is measured from ground level, parapets and plant enclosures count, and a survey that is 300 mm out can move a scheme from compliant to not.

Matters the authority may consider on prior approval

Prior approval is not a formality and it is not an open planning assessment either. The authority is confined to the matters the Order lists, which for the Part 20 classes include transport and highways impacts, air traffic and defence asset impacts, contamination and flood risk, the external appearance of the building, the provision of adequate natural light in all habitable rooms of the new dwellings, the impact on the amenity of the existing building and neighbouring premises including overlooking, privacy and loss of light, and the impact of construction on the occupiers of the building and of neighbouring premises.

Two of those are doing most of the work in practice.

Adequate natural light in all habitable rooms is a mandatory consideration and a common reason for refusal. A rooftop flat with a single window serving a deep plan living space will struggle, and the Order treats the absence of adequate natural light as a ground to refuse rather than as a matter of judgement. A Daylight and Sunlight Assessment addressing both the new units and the effect on neighbours is worth preparing before submission rather than in response to a refusal.

External appearance gives officers a genuine design say, which surprises applicants who expected a tick box. A badly detailed additional storey in a visible location can be refused on appearance alone.

Where the right does not apply

The exclusions are extensive and each one is absolute.

• land within a conservation area, a National Landscape, a National Park, the Broads or a World Heritage Site
• listed buildings and land within their curtilage, and scheduled monuments
• sites of special scientific interest, safety hazard areas and military explosives storage areas
• buildings constructed outside the eligible date window, which begins on 1 July 1948 and closes on a date that differs between the classes
• buildings where an Article 4 direction has withdrawn the right
• schemes that would breach any of the conditions or limitations in the class relied on

Check the current text of the Order rather than a summary, because Part 20 has been amended since it was introduced and the detail of the date windows and the conditions matters.

Why many schemes still need a full application

The honest summary is that prior approval is attractive where the building and the scheme fit the class cleanly, and a trap where they nearly fit. A scheme designed to squeeze inside the caps often produces poor flats with inadequate light, which then get refused on a mandatory consideration.

A full application costs more and takes longer and buys freedom: a third storey where the class permits two, a set back that improves the appearance, larger windows, a different mix. On a constrained urban block that freedom is frequently worth more than the procedural saving. Where the application is contentious, whether it is decided by officers or by members matters, and the difference between committee determination and a delegated decision is worth understanding early.

Structural and fire consequences

Building upwards loads an existing structure that was designed for the building as it stood. The existing frame, walls and foundations have to be checked, and the Order's allowance for strengthening works is there because strengthening is usually needed.

A Structural Survey establishing the existing capacity is the first technical appointment, before the architect draws the new storeys. Where the existing foundations cannot take the load, the options are underpinning, a transfer structure or a lighter construction for the new storeys, and all three change the scheme.

Fire is the other threshold. Where the building as extended reaches 18 m or seven storeys and contains two or more dwellings, it becomes a relevant building for Planning Gateway One, and a Fire Statement is required under the Building Safety Act 2022 framework and article 9A of the Development Management Procedure Order 2015. That document has to be prepared by someone with the necessary fire engineering competence, which is a legal requirement rather than a recommendation, and an AI-assisted draft is a starting point for that professional and not a substitute.

Adding storeys also changes means of escape, the single stair question and the existing building's compartmentation, none of which is a planning matter and all of which can make the scheme undeliverable.

Evidence for each route

For prior approval, the submission needs to prove compliance with the class and address every listed matter: an accurate measured survey with heights, existing and proposed drawings and elevations, a daylight assessment for the new units, an amenity assessment for neighbours, a construction management report, and a Transport Statement or parking position where highways is engaged.

For a full application the pack is the usual one, with design and heritage material doing more work, and affordable housing and policy compliance addressed.

Either way, commission the structural appraisal first. It is the cheapest way to find out whether the scheme exists at all.

This is general guidance, not legal advice, and permitted development rights are amended frequently. A lawful development certificate is the only formal way to confirm that works are lawful. Check the current text of the Order and the position with your local planning authority or a qualified professional before relying on it.

Enter your postcode on PlanScope to see which reports an upward extension is likely to need. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.