Part 20 upward extensions: adding storeys by prior approval

Part 20 of Schedule 2 to the General Permitted Development Order 2015 contains five upward extension classes, and picking the wrong one is the most common reason a part 20 upward extension notification fails. Each class covers a different building type, sets a different storey allowance and a different overall height cap, and applies its own construction date test. Read the class you are relying on in the Order itself rather than a summary, because the classes were introduced and amended at different times and the differences between them are not intuitive.

The other thing to know early is that the planning route is the easy part. Ownership of the airspace and the structural and fire consequences of adding two storeys are where these schemes actually stall.

The classes and the buildings they cover

Class A covers new dwellings built on top of a detached purpose-built block of flats.

Class AA covers detached buildings in commercial or mixed use. Class AB covers terrace buildings in commercial or mixed use, and terrace buildings in use as dwellinghouses. Class AC covers houses in a terrace. Class AD covers detached houses.

The distinction between detached and terraced runs through the whole of Part 20 and it decides both which class applies and how much height you get. A semi-detached house is treated as a terrace building for these purposes, which catches out owners who assume the detached class applies because their house has a gap on one side.

Class ZA sits in the same Part and does something different: demolition of a qualifying building and its replacement with homes. It is not an upward extension right.

Storey and height limits

Height is only half the constraint. New storeys must sit immediately above the topmost storey of the existing building, and the floor to ceiling height of each new storey is limited to three metres or the floor to ceiling height of any existing storey, whichever is less. On a 1960s block with 2.4 metre storey heights, that is the number you design to.

The roof pitch of the extended building has to match the existing roof, and the engineering operations permitted are limited to strengthening existing walls and foundations and installing services. Anything beyond that falls outside the class.

None of these rights apply on article 2(3) land, so a conservation area, National Park, National Landscape, the Broads or a World Heritage Site rules them out, as do listed buildings and their curtilage, scheduled monuments, sites of special scientific interest, safety hazard areas and military explosives storage areas.

Construction date tests

Each class limits itself to buildings constructed within a defined window. For the terrace and detached house classes and the commercial classes the window runs from 1 July 1948 to 28 October 2018. For Class A the closing date differs, and 5 March 2018 is the date to check.

Evidence the date. Historic mapping, the original building control record, a rating record or an early aerial photograph will usually settle it. A building substantially rebuilt after the closing date may fall outside the class even though the original structure predates it, and that is a question worth resolving before design fees are spent.

The prior approval matters

Part 20 gives the council a list of matters on which prior approval must be sought, and the list varies between classes. Common ground across them includes transport and highways impacts, air traffic and defence asset impacts, contamination risk, flooding risk, the external appearance of the building, the provision of adequate natural light in all habitable rooms, and the impact on the amenity of the existing building and of neighbouring premises including overlooking, privacy and loss of light. In London, protected vistas can be added.

Natural light is a hard condition rather than a matter of degree. Every habitable room in the new dwellings needs adequate natural light, and a deep plan on a constrained site can fail it. A Daylight and Sunlight Assessment using current BRE guidance answers both that condition and the amenity question for the flats below and next door, which is the objection neighbours raise first.

New dwellings created under Part 20 are also expected to meet the nationally described space standard, so check the conditions in the current text of the Order alongside the limitations.

Construction impact is the matter applicants underestimate. Living above an occupied block for a year of works generates complaints, and a Construction Management Plan setting out working hours, deliveries, scaffolding, noise and dust control and a resident liaison route is the document that makes an approval workable.

Structure, fire and the Building Safety Act

Adding two storeys to a building designed in 1965 for four is a structural question before it is a planning question. Foundation capacity, lateral stability, the existing frame and the condition of the roof structure all need assessing, and lightweight modular construction is often the only route that works. A Structural Survey by a chartered engineer, done before the notification, tells you whether the scheme exists at all.

Fire is the constraint that changes the project. If the extended building reaches 18 metres or seven storeys it becomes a higher-risk building under the Building Safety Act 2022 framework, with the Building Safety Regulator's gateway regime applying. That is a different programme, a different cost base and a different design team.

Where a Fire Statement is required for a relevant building, it must be prepared by someone with the necessary fire engineering competence. Under Article 9A of the Development Management Procedure Order 2015 and the Planning Gateway One arrangements this is a legal requirement, and a Fire Statement drafted without that competence is a starting point for the fire engineer rather than a substitute for one. Check with the local planning authority how the requirement applies to a prior approval application for your building, because the position differs from a full planning application.

Airspace, leases and who owns the roof

Permitted development gives you a planning route. It gives you nothing in property terms.

In a block of flats the roof and airspace may be demised to a leaseholder, retained by the freeholder, or subject to rights that prevent building over. Where a landlord proposes to dispose of an interest in airspace in a building with qualifying tenants, the right of first refusal under the Landlord and Tenant Act 1987 may apply, and getting that wrong is expensive.

Access for construction, crane oversail, scaffold licences and party wall notices under the Party Wall etc. Act 1996 all need resolving alongside the planning work. So does the practical question of how residents reach their front doors during a build over their heads.

Take legal advice on title and leases before the notification, not after.

What a complete notification contains

1. Existing and proposed plans, elevations and sections, with storey heights and overall height dimensioned.
2. Evidence of the construction date of the existing building.
3. A statement taking each limitation and condition in the relevant class in turn and showing it is met.
4. Daylight and amenity evidence for the new dwellings and for neighbouring occupiers.
5. Contamination, flood risk and highways material proportionate to the site.
6. A construction management plan covering the occupied building below.
7. Confirmation of the notices served on owners and occupiers, and the site notice.

This is general guidance, not legal advice, and a Lawful Development Certificate is the only formal way to confirm that works are lawful. Check the position with your local planning authority or a qualified professional before relying on it.

To scope the reports a Part 20 scheme needs, browse the PlanScope report types.

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