Class AA (Part 20): Adding New Homes Above Detached Commercial and Mixed-Use Buildings

If you own a detached commercial or mixed-use building, permitted development rights under Part 20, Class AA of the Town and Country Planning (General Permitted Development) (England) Order 2015 (GPDO 2015) may allow you to construct up to two additional storeys of new flats above it — without making a full planning application.

For investors and landowners, this is one of the most commercially significant airspace development rights on the statute book.

This article explains what Class AA permits, the limitations that apply, the prior approval matters the LPA will assess, and the practical issues you need to resolve before progressing a scheme.

What Does Class AA Permit?

Class AA permits works for the construction of up to two additional storeys of new dwellinghouses — specifically flats — immediately above the topmost storey of a qualifying detached building. The new homes must be constructed as flats: Class AA does not permit the creation of a single house occupying the new storeys.

The right extends to associated works reasonably necessary to deliver the new homes, including:

• Engineering operations to construct the additional storeys (including strengthening existing walls and foundations, and installation or replacement of services)

• Replacement or installation of plant on the roof of the extended building, where reasonably necessary to service the new flats

• Works to provide appropriate access and egress, including means of escape from fire, via additional external doors or external staircases

• Storage, waste and other ancillary facilities reasonably necessary to support the new dwellinghouses

This is a broad right that allows a developer to deliver a complete residential scheme above an existing commercial building — including staircases, plant replacement and bin stores — without needing separate consents for each element.

Which Buildings Qualify?

The building must be a detached building. This is the key distinction from Class AB (which applies to terraced and semi-detached commercial buildings). A building is detached if it is not joined to any other building at the time the application is made.

The existing building must, on 5 March 2018, have been in use for one or more of the following purposes (which should be read by reference to the pre-2020 Use Classes Order):

• Class A1 (shops)

• Class A2 (financial and professional services)

• Class A3 (restaurants and cafes)

• Class B1(a) (offices)

• Betting office, pay day loan shop, or launderette

• A mixed use combining any two or more of the above

• A mixed use combining any of the above with Class C3 (dwellinghouses)

The inclusion of mixed-use buildings — including buildings already containing residential units — is commercially significant. A detached building that was, in March 2018, operating as a ground-floor shop with a flat above qualifies under Class AA, making this right particularly relevant to the large stock of traditional high street properties across England.

The building must also have been constructed on or after 1 July 1948 and on or before 5 March 2018.

Key Limitations

Class AA is subject to a number of important limitations, all of which must be satisfied before the PD right applies. Exceeding any single limitation renders the entire development unlawful — there is no de minimis principle.

Height of existing building

The existing building must be at least three storeys in height above ground level. This is a hard threshold: a two-storey commercial building cannot benefit from Class AA, regardless of how it might compare in height to neighbouring buildings.

Height of the extended building

The height of the highest part of the roof of the extended building must not exceed the height of the highest part of the existing roof by more than 7 metres. The height of the highest part of the roof of the extended building (excluding plant) must not exceed 30 metres.

Floor to ceiling height of new storeys

The floor to ceiling height of any additional storey, measured internally, must not exceed the lower of 3 metres or the floor to ceiling height of any storey in the principal part of the existing building. This prevents the new storeys from appearing disproportionately tall relative to the existing building.

The principal part

The additional storeys must be constructed on the principal part of the building — the main structural mass, excluding front, side or rear extensions of lower height. Where a detached commercial building has an attached lower-height rear extension, additional storeys can only be built above the principal mass, not above the extension. This mirrors the definition of "principal part" used in Part 1, Class AA for dwellinghouses.

No visible support structures

On completion of the development, there must be no visible support structures on or attached to the exterior of the building. This reflects the policy intent that the extended building should appear as a coherent whole, with any structural works concealed within the envelope of the development.

Engineering operations

Permitted engineering operations are limited to works within the existing curtilage of the building to: strengthen existing walls; strengthen existing foundations; or install or replace services. Operations that extend beyond the curtilage are not PD, nor are ground-works that do not fall within these categories.

Plant

Where Class AA development includes replacement or additional plant on the roof, the height of the replaced or additional plant (measured from the lowest surface of the new roof on the principal part of the extended building) must not exceed the height of any existing plant measured from the lowest surface of the existing roof. Class AA development is not permitted if there is no existing plant on the building at all.

Location restrictions

Development is not permitted where the site is on Article 2(3) land (National Parks, Areas of Outstanding Natural Beauty, Conservation Areas, World Heritage Sites and the Broads), a Site of Special Scientific Interest, within the curtilage of a listed building, on or within the curtilage of a scheduled monument, in a safety hazard area, in a military explosives storage area, or within 3 kilometres of the perimeter of an aerodrome.

Prior Approval: What the LPA Will Assess

Unlike some Class A or B permitted development rights, Class AA requires a mandatory prior approval application. Development must not begin until written notice of the grant of prior approval has been received from the LPA. There is no deemed grant mechanism for Part 20 — if the LPA fails to determine the application, the only route is to appeal.

The prior approval matters the LPA will assess are:

• Transport and highways — the impact of the development on the local road network and highway safety

• Contamination risks — whether the site is or may be contaminated

• Flood risk — the risk of flooding on the site

• External appearance — the design and appearance of the proposed storeys and the impact on the existing building and its surroundings

• Amenity — the impact on the amenity of the existing and neighbouring uses, including overlooking, privacy and loss of light

• Natural light — the provision of adequate natural light in all habitable rooms of the proposed new dwellinghouses. Prior approval must be refused if adequate natural light cannot be demonstrated

• Impact on the character or use of the existing building — whether the proposed development is consistent with the building remaining in its existing lawful use

• Provision of adequate natural light — as noted above, this is a mandatory ground for refusal if not met

The LPA must determine the application within 56 days of receiving a valid application. The application must be accompanied by a written description of the proposed development, floor plans and elevations, a statement of the net increase in dwellinghouses proposed, and the correct fee.

Space Standards and Minimum Size

From 6 April 2021, Article 3(9A) of the GPDO provides that Schedule 2 does not permit any new dwellinghouse created under Class AA where the gross internal floorspace is less than 37 square metres, or where the dwelling does not comply with the nationally described space standard issued by DCLG on 27 March 2015. This applies to every new flat created — not just the scheme as a whole.

In practice, this means that the floor-to-ceiling height restrictions and the 37m² minimum combined set a meaningful constraint on the design of new airspace flats. A scheme that creates new flats falling below the minimum size will be refused, even if all other prior approval matters are satisfied.

Practical Considerations

Structural viability — before making a prior approval application, a structural assessment of the existing building is essential. The permitted engineering operations are limited in scope, and a building that requires more extensive works to support additional storeys may not be able to deliver those works within the PD right.

The existing building must remain in use — Class AA is designed to enable new homes above existing commercial and mixed-use buildings, not to enable the conversion of those buildings. The prior approval matters include an assessment of the impact on the character or use of the existing building. A scheme that effectively prevents continued commercial use of the ground floor is unlikely to secure prior approval.

No Article 4 Direction — Article 4 Directions can remove Part 20 PD rights. Some LPAs have made directions removing Class AA rights, particularly in employment areas and town centres. This must be confirmed before any application is made.

Building Regulations — prior approval is separate from Building Regulations approval. All Class AA developments will require Building Regulations consent, including compliance with Part B (fire safety) and Part M (accessibility). The access and egress requirements in the PD right — including means of escape from fire — must be addressed in the prior approval application and in the subsequent Building Regulations application.

The s78 appeal route — where prior approval is refused or granted subject to unreasonable conditions, an appeal can be made under section 78(1)(c) of the TCPA90. Given the absence of a deemed grant for Part 20, it is important to submit a properly evidenced application first time. From April 2026, amended plans and schemes cannot be submitted once an appeal has been lodged.

Summary

Class AA is a commercially attractive right for owners of qualifying detached commercial buildings, particularly traditional high street properties where the upper airspace has historically been underused. The right to add up to two storeys of flats — including access, plant and ancillary facilities — without a full planning application is a significant opportunity.

However, the limitations are detailed, the prior approval matters are substantive, and there is no fallback deemed grant if the LPA fails to determine in time. Professional advice before submission is essential.

Get Prior Approval Planning Advice

If you are considering a project that may benefit from the prior approval route, get in touch for clear, practical advice on your options.

Andrew Ransome MRTPI Email: andrew@adpltd.co.uk | Tel: 01206 242070

About me

Andrew Ransome is the planning director at ADP and is a chartered member of the RTPI, with over 22 years of town planning experience.

Andrew has extensive experience offering strategic planning solutions to challenging projects in both rural and urban settings. Follow him on Linkedin.

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