Part 20 upward extensions and the classes that allow them

Part 20 of Schedule 2 to the General Permitted Development Order 2015 contains several separate classes permitting additional storeys, and the one you need depends on what the existing building is and what it is used for. Get the class right first, because the qualifying dates, the height limits and the number of storeys allowed all differ between them.

For a landlord holding a block of flats or a parade with flats above, the upward extension right looks attractive on paper. In practice the leasehold structure defeats it more often than the planning limits do.

Which classes permit additional storeys

The classes divide by building type and by existing use.

Do not confuse these with Class AA of Part 1, which is the householder right to add a storey to a house and has its own separate limits and procedure. The Part 20 classes create new dwellings; the Part 1 right enlarges an existing one.

Terrace has a specific meaning in the Order, covering a building in a row of three or more, and it includes an end of terrace building. A semi-detached pair is neither detached nor a terrace for these purposes, which removes a large slice of the housing stock from the classes altogether.

Building age and use conditions

Each class carries a construction date window, and the windows are not the same. The block of flats class uses one cut-off date and the commercial and dwellinghouse classes use another, both starting from 1 July 1948.

A building constructed after the relevant cut-off does not qualify, however suitable it is. Neither does a building that was constructed earlier but has been substantially rebuilt since, where the evidence shows the rebuild is the building that now stands.

Use matters as much as age. The commercial classes require the building to have been in a qualifying use, which centres on the broad commercial class introduced in 2020 together with certain named uses. Gather the evidence for both limbs before you apply: building control records, historic maps, rating records, leases and photographs.

Read the current text of Part 20 rather than a summary, including this one. The Part has been amended more than once since 2020, and the dates and limits are the whole of the deal.

Height limits and the relationship to neighbours

Three separate height controls operate together, and a scheme has to satisfy all of them.

• An overall cap on the height of the resulting building, which differs between the block of flats class and the other classes
• A cap on how far the resulting building may exceed the height of the highest part of the roof of the existing building
• A limit on the floor to ceiling height of each additional storey, measured internally

There is also a control linking the new height to neighbouring buildings in a terrace, which prevents an extended building standing far above the roofline of those next to it. On a terrace of consistent height that control is usually the binding one.

Measure the existing building properly before you design. Roof height is taken from the highest part of the existing roof excluding plant, and on a block with a flat roof, lift overruns and tank rooms the answer is not obvious from a drawing produced in 1966.

Prior approval matters and external appearance

The list of matters is long enough that the process feels like a planning application. Expect the authority to consider transport and highways, air traffic and defence assets, contamination, flooding, the external appearance of the building including the design of the principal elevation and any side elevation fronting a highway, the amenity of the existing building and neighbouring premises, protected views, air quality for future residents, and the impact of construction on the area.

External appearance gives officers real discretion on a prominent building. A two-storey addition in a contrasting material, set flush with the existing facade, invites a refusal on appearance where a set-back storey in a recessive finish does not.

Construction management is also assessed, so submit a document dealing with hours, deliveries, scaffolding, protection of existing occupiers and waste, rather than leaving it to a condition.

Where the right is unavailable, it is usually because of designation. The classes do not apply on article 2(3) land, which covers conservation areas, National Parks, National Landscapes, World Heritage Sites and the Broads, nor to listed buildings and their curtilage, scheduled monuments or sites of special scientific interest. An Article 4 direction can remove the right locally, and how Article 4 directions work against the national policy test is worth understanding before you buy on the strength of Part 20.

Daylight as a prior approval matter

The provision of adequate natural light in all habitable rooms of the new dwellings is a specific prior approval matter, and it is one of the more effective grounds for refusal.

That bites in two places. New flats formed at roof level with limited glazing on one aspect, particularly where a mansard form reduces window area, can fail on their own daylight. And the effect on neighbouring premises, where an added storey overshadows windows opposite or reduces vertical sky component at an adjoining flat, is assessed as an amenity matter.

Commission a Daylight and Sunlight Assessment against the recognised methodology, covering both the proposed units and the affected neighbours, and design to the results rather than submitting and hoping. New dwellings created under permitted development must also meet the nationally described space standard, so the internal layout has to work at the same time.

Structural and fire consequences

Adding two storeys to a building designed for its original height is a structural exercise, and the answer may be a lightweight frame, new foundations, or that it cannot be done at all. Commission a Structural Survey and an appraisal by a chartered engineer before the design is fixed.

Fire is the bigger programme risk. Taking a building above the statutory thresholds for a higher-risk building brings it within the regime created by the Building Safety Act 2022, with its own gateway approvals and a different construction process. Planning Gateway One also applies to relevant buildings at the planning stage, which means a Fire Statement prepared by someone with the necessary fire engineering competence, and consultation with the Building Safety Regulator.

A fire statement is a document where a competent professional is legally required. An AI-assisted draft is a starting point for that professional rather than a substitute for them.

Why leasehold structures often defeat the right

Planning permission granted by the Order says nothing about who owns the airspace or who is entitled to build in it.

Four property issues recur. The roof or roof void may be demised to top-floor leaseholders rather than retained by the freeholder. Leases commonly contain covenants for quiet enjoyment and against alterations that the works would breach. Where the freeholder proposes to dispose of the airspace, leaseholders may have statutory rights that have to be dealt with first. And a building where leaseholders have acquired the freehold collectively, or exercised the right to manage, has a different decision maker from the one on the title when you first looked.

Resolve the property position before spending money on the planning application. A prior approval you cannot implement is worth nothing, and the three-year completion condition attached to the classes will run out while the lawyers argue.

Following how these classes have been amended is easier through the planning law updates timeline than through the Order itself, and the same is true of related changes such as Class MA after the removal of the floorspace cap.

This is general guidance rather than legal advice. Check the current text of Part 20 and 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. For specialist technical reports, qualified professional input may be required.