Upward extensions under Part 20 and the prior approval tests

A Part 20 upward extension prior approval application is not a light-touch route. The classes in Part 20 of Schedule 2 to the General Permitted Development Order grant the principle of adding storeys, which is a real advantage, and then hand the authority a list of prior approval matters wide enough to refuse most schemes that have not been designed for them. Daylight to the new flats, amenity of the existing occupiers, external appearance, transport, contamination, flooding and construction management are all in play. The principle is granted. Almost nothing else is.

Read the current text of Part 20 before you design. These classes have been amended since they were introduced and the limits are precise.

Which classes apply to which buildings

Part 20 contains several classes, and picking the wrong one wastes an application fee.

The word detached does a lot of work. A block of flats in a terrace is not within Class A, and a semi-detached house is treated as a terrace building for these purposes rather than as detached. Establish the building's typology from the site survey, not from the estate agent's description, because an application made under the wrong class is invalid rather than merely weak.

The classes also differ in what they produce. Classes A, AA and AB create new flats. Classes AC and AD extend or create dwellings on houses, with additional restrictions on what the new accommodation can be.

Height limits and the reference buildings

Three limits operate together and all three have to be satisfied.

A cap on the number of additional storeys, which is two where the existing building has two or more storeys and one where it has a single storey. A cap on the total height of the resulting building, which is 30 metres for a detached block of flats under Class A and 18 metres for the other classes. And, for the dwellinghouse classes, a cap on the increase over the existing roof height, which is 3.5 metres for one storey and 7 metres for two.

For a terrace, there is a further comparison with the neighbouring buildings, so the roofline of the terrace constrains what a single plot can do. That is the limit that most often kills a scheme on a Victorian terrace with varied parapet heights, and it has to be established by measured survey of the adjoining properties rather than assumed.

The construction date of the existing building matters as well. Class A applies only where the block was constructed between 1 July 1948 and 5 March 2018, which excludes pre-war mansion blocks and recently completed buildings alike. Check the date from the authority's records or from the building's own history, and be ready to evidence it.

Internal floor-to-ceiling heights of the new storeys are also limited, which interacts with the structural depth you need and with the services zone. On a flat-roofed 1960s block that is often the binding constraint rather than the overall height cap.

What the authority may consider at prior approval

The prior approval matters are the application. Expect the authority to require evidence on each of them, and to refuse where evidence is absent rather than to assume in your favour.

• Transport and highways impacts, including parking and servicing for the new units.
• Air traffic and defence asset impacts, which matter near aerodromes and safeguarded sites.
• Contamination risk affecting the building.
• Flooding risk affecting the building.
• The external appearance of the building, including the design of the principal elevation and any side elevation fronting a highway.
• Adequate natural light in all habitable rooms of the new dwellings.
• The amenity of the existing building and neighbouring premises, including overlooking, privacy and loss of light.
• Protected vistas, where the London directions apply.
• Noise from commercial premises affecting the new occupiers, for the commercial and mixed use classes.
• The effect of introducing residential use where the authority considers the area predominantly commercial.

Natural light is the one that catches designers out. It is both a prior approval matter and the subject of a specific requirement, and a single-aspect flat with a deep plan and a rooflight to a bedroom will not satisfy it. Commission a Daylight and Sunlight Assessment that covers both the new units internally and the existing flats below, because the second half is what the existing residents will object about.

New dwellings created under permitted development have had to meet the nationally described space standard since April 2021. That changes the unit mix available on a given roof plate and it is not negotiable.

Documents the application has to contain

The prior approval application is a defined submission and an incomplete one does not start the clock. The Order requires, in broad terms, a plan showing the site, a written description of the proposed development, drawings of the existing and proposed elevations, a developer's report dealing with the management of construction, and the fee.

The construction management report is the document most often treated as a formality and most often used as a reason for refusal. On an occupied block it has to deal with working hours, access for plant, scaffold and crane positions, how residents will reach their front doors, dust, noise and the programme. Write it as though the residents will read it, because they will.

Where a structural intervention is needed at the existing frame or foundations, a Structural Survey is worth commissioning before the prior approval application rather than after, because its findings can change the number of storeys that is feasible and therefore the application itself.

Exclusions that take a building out of Part 20

Part 20 does not apply on article 2(3) land, which covers conservation areas, National Parks, National Landscapes and World Heritage Sites. It does not apply to a listed building or within its curtilage, nor to a scheduled monument or its curtilage. Sites of special scientific interest, safety hazard areas and military explosives storage areas are also excluded.

An Article 4 direction can withdraw the rights in a particular area, and some authorities have made directions aimed specifically at upward extensions. Check the direction register as well as the policies map, and remember that an application needed only because a direction removed the right attracts no fee. The same fee point applies to other withdrawn rights, as our note on air source heat pumps and permitted development limits explains.

Development under the General Permitted Development Order also sits outside the mandatory biodiversity net gain regime, which is part of the commercial attraction of these classes. Confirm the current position before relying on it, because the exemptions have been adjusted.

Structural and fire consequences of adding storeys

Two additional storeys on a 1960s block is a serious structural proposition. The existing frame was designed to the loadings of its day, the foundations were sized for the building as built, and lateral stability for a taller building is rarely a simple addition. Light steel or timber framing above a transfer structure is the usual answer and it changes the floor-to-ceiling arithmetic.

Fire is the more consequential issue. Where the resulting building reaches 18 metres or seven storeys it falls within the higher-risk building definition, which brings the Building Safety Act gateways for the construction phase and changes the design approach to means of escape, external wall construction and firefighting access. Whether the planning fire statement requirement under Article 9A of the Development Management Procedure Order applies to a prior approval application rather than to an application for planning permission is a question to settle with the authority in writing at the outset.

Adding storeys also affects the existing building's fire strategy. A single stair that was acceptable at four storeys may not be at six, and the resolution is a fire engineering exercise rather than a planning one. For specialist technical reports, qualified professional input may be required, and a fire safety strategy is a fire engineering document that a competent professional must prepare.

Why many of these schemes end up as full applications

The honest position is that Part 20 works well for a narrow set of buildings and poorly for the rest. Flat-roofed detached blocks from the 1960s and 1970s, outside designated land, with generous roof plates and a simple structural grid, are the cases where it delivers.

Elsewhere, four things push schemes back to a full application: the terrace roofline comparison, the external appearance test where the authority wants a different architectural response, natural light in deep plans, and a design that needs to step beyond the existing footprint for stair or lift access.

A full application is slower and attracts policy requirements that Part 20 avoids, including affordable housing on larger schemes. It also allows a design that works. Run both routes in parallel at feasibility stage, price the difference, and make the choice on the drawing rather than on the principle.

Follow amendments on the planning law timeline, and browse the report types a prior approval application typically needs. This is general guidance, not legal advice. The limits in Part 20 are precise and have been amended, so check the current text of the Order, and a Lawful Development Certificate is the only formal way to confirm that works are lawful.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.