Permission for additional storeys under Part 20

Part 20 of Schedule 2 to the General Permitted Development Order 2015 allows new homes to be built on top of existing buildings, subject to prior approval. For a landlord holding a 1960s block of flats or a parade with flats above shops, it is the quickest route to additional units on land already owned. It is also the route with the highest refusal rate, because the prior approval matters are wide enough to cover most of what a full application would consider.

The decision is therefore not whether Part 20 is available, but whether the prior approval matters can be answered more easily than a planning application could be won.

Which buildings the class applies to

Part 20 contains several classes, and they apply to different building types with different limits:

• Class A: new flats on detached blocks of flats
• Class AA: new homes on detached buildings, or terraces, in commercial or mixed use
• Class AB: new homes on terraced buildings in use as dwellinghouses
• Class AC: new homes on semi-detached houses
• Class AD: new homes on detached houses

Each class carries its own construction date window, its own number of permitted additional storeys and its own height ceilings, and the differences are not intuitive. Read the class that applies to your building in the Order rather than working from a summary, including a summary like this one.

The exclusions are common to the classes and they are extensive. Land in a conservation area, a National Park, a National Landscape, a World Heritage Site or the Broads is outside the right. So is a listed building and land within its curtilage, a scheduled monument, a site of special scientific interest, a safety hazard area and a military explosives storage area. Check the designation position before spending anything on design.

Height limits and the reference building

Three constraints work together in every class and they have to be tested against measured survey data rather than an old drawing.

The first is the number of additional storeys, which is one or two depending on the class. The second is a ceiling on the height of the resulting building, measured from ground level, which is higher for blocks of flats than for houses. The third caps how far the new storeys may project above the highest part of the existing roof, excluding plant.

There is also a floor to ceiling limit on each new storey, which tends to be the constraint that defeats a scheme on paper. A developer assuming a 3 m floor to ceiling height, plus floor build-up, plus a parapet, frequently finds the resulting height exceeds one of the other limits.

Measure the existing building properly first. Ground level is defined in the Order and on a sloping site the measurement point matters, which is where several prior approvals fail on a technicality that a topographical survey would have caught.

Prior approval matters the authority may consider

The list is long, and each item is a ground on which prior approval can be refused:

External appearance is the one that surprises people, because it brings design squarely within the authority's reach. A poorly resolved rooftop addition on a visible elevation can be refused on appearance alone, so the design has to be as considered as it would be for a full application.

Natural light applies to all habitable rooms, as it does under the other residential conversion rights, and a Daylight and Sunlight Assessment is worth commissioning where any proposed room is marginal or where the addition will affect the floor below.

Structural and fire information required

The Order specifies the information that must accompany the application, and for upward extensions that includes engineering detail about the works to the existing structure. Supply it properly: a structural appraisal of the existing building, the proposed means of support, and the implications for foundations.

Fire safety is a separate consent stream that cannot be left until later. Where the resulting building reaches 18 m in height or seven or more storeys, it becomes a relevant building for the purposes of the planning gateway, and a Fire Statement prepared by someone with the necessary fire engineering competence is required. That is a legal requirement rather than a preference, and the consultee engagement adds time to the programme.

Even below that threshold, adding storeys changes escape distances, stair capacity and compartmentation for the whole building, not only for the new part. Resolve it with a fire engineer before the layout is fixed.

Amenity of existing occupiers during and after works

This is the matter that generates objections and the one landlords underestimate. Existing leaseholders and tenants live in the building throughout construction, and the authority may consider the impact of the works on them.

A construction management plan that addresses working hours, scaffold arrangements, access to entrances and refuse stores, noise, dust and the sequencing of roof removal is the document that answers it. Offer it with the application.

After completion, the questions are about shared services, lifts, bin stores, cycle parking and whether the existing provision can absorb additional households. Where the lift does not serve the new storeys, or the bin store is already undersized, expect that to be raised.

The leasehold and landlord and tenant position is a separate matter from planning, and it has defeated more rooftop schemes than the planning system has. Check the demise, the roof ownership and any restrictive covenant before applying.

Common reasons prior approval is refused

Six recurring grounds:

1. The scheme exceeds one of the height limits, usually once floor build-up and parapets are added.
2. Habitable rooms with inadequate natural light, particularly in deep plans.
3. External appearance, where the addition reads as an afterthought on a visible elevation.
4. Overlooking of neighbouring properties from new windows and terraces.
5. Insufficient information, particularly on structure, drainage or refuse.
6. The building or land falls within one of the excluded categories.

Ground five is the avoidable one. The Order sets out what must be submitted and authorities refuse incomplete applications rather than asking for more, because the determination period is fixed.

Programme from application to start on site

The prior approval period is 56 days. Alongside it, several items run in parallel and should start earlier:

• topographical and measured building survey, before any design work
• structural appraisal and proposed support strategy
• fire engineering input, and gateway engagement where the height threshold is reached
• notification of existing occupiers and early engagement with leaseholders
• levy liability assessment, since new dwellings created this way are chargeable
• building control route and the pre-construction information

The levy point catches people. Additional dwellings are chargeable development, and the liability has to be assumed and the commencement notice served before work starts, or reliefs are lost.

Where a full application is the better route

Part 20 is not always the right answer. A full application is usually better where the design wants more than two storeys, where the building sits in a designated area, where the layout needs changes to the existing floors, or where the scheme benefits from being presented as a whole with a proper design narrative.

The comparison between the rooftop development right for commercial buildings and a full application is worked through in our note on rooftop flats under Class AA compared with a full application, and the same reasoning applies to the residential classes.

Where a full application is chosen, the narrative sits in a Planning Statement and a Design and Access Statement, and the design freedom usually more than compensates for the longer determination period.

This is general guidance, not legal advice; check 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.