Upward extensions of blocks and terraces under Part 20

Upward extension permitted development for flats sits in Part 20 of Schedule 2 to the GPDO 2015, and it is the most conditional part of the Order. The classes cover detached blocks of flats, terrace buildings in commercial or mixed use, terrace buildings in residential use and detached houses, with a separate class permitting demolition and replacement. Each one has its own construction date condition, its own storey and height limits and its own prior approval list.

Before you draw anything, establish three facts about the building: what it is, when it was built and whether the land is excluded. Two of those three kill most schemes.

The construction date condition

Part 20 rights depend on when the building was constructed, and the ranges differ between classes. For a detached purpose built block of flats the building must have been constructed between 1 July 1948 and 5 March 2018.

That range does real work. A 1930s mansion block is outside it. So is a block completed in 2019. Evidence of the construction date therefore becomes a submission document in its own right, and the usual sources are historic Ordnance Survey mapping, building control records, the original planning permission and rating records.

Purpose built also matters. A former office converted to flats is not a purpose built block of flats, however convincing the conversion, and a scheme relying on the wrong class fails at the first legal challenge rather than at prior approval.

Check the current consolidated text of Part 20 for the class that matches your building, because the classes and their conditions have been amended since 2020.

Storey and height limits and how they are measured

Three limits usually apply to an upward extension of a block.

The number of additional storeys is capped, commonly at two for a building of three or more storeys and one for a lower building. The additional storeys must not exceed 7 metres above the highest part of the existing roof, measured excluding plant. And the extended building as a whole must not exceed 30 metres in height.

Internal floor to ceiling heights are also controlled. Each new storey must not exceed 3 metres, or the floor to ceiling height of any existing storey, whichever is the lower. On a 1960s block with 2.4 metre storeys, that is the constraint that decides whether the scheme works, because it caps the new floors at the existing dimension rather than at a comfortable modern one.

Measure from the existing building as built rather than from the drawings. Roof finishes, upstands and existing plant enclosures all affect where the datum sits, and a measured survey is cheaper than a refused prior approval.

Excluded land removes the right entirely. That includes article 2(3) land such as conservation areas, National Landscapes, National Parks, the Broads and World Heritage Sites, along with sites of special scientific interest, listed buildings and land within their curtilage, scheduled monuments, safety hazard areas and military explosives storage areas.

The prior approval matters

Part 20 prior approval is wide compared with other classes. The matters typically listed include transport and highways impacts, air traffic and defence asset impacts, contamination risk, flooding risk, the external appearance of the building including the design of the principal elevation and any side elevation fronting a highway, the provision of adequate natural light in all habitable rooms, the impact on the amenity of the existing building and neighbouring premises including overlooking, privacy and loss of light, and the impact on any protected view.

Two of those carry most of the refusals.

Natural light is assessed for every habitable room in the new dwellings, and a single aspect flat behind a deep plan with a recessed balcony will struggle. Design the daylight in rather than defending it afterwards.

Amenity of the existing building is the other. Residents below gain a construction programme and lose their roof terrace, their lift capacity and sometimes their daylight. Authorities take that seriously, and a scheme that ignores the existing occupiers reads badly.

Bring a construction management statement with the application. Hours of work, deliveries, scaffolding, crane positions and how the existing residents keep access are all matters authorities expect to see addressed.

Fire safety and the Gateway One interaction

Fire safety has been drawn into this area from two directions. The prior approval process for upward extensions has been amended to deal with fire safety, and the planning system separately requires a fire statement for relevant buildings under Article 9A of the Development Management Procedure Order 2015, with the Building Safety Regulator as a consultee at Gateway One.

Whether your scheme engages the Gateway One process depends on the height and composition of the building after extension, and adding two storeys is exactly the sort of change that can cross the threshold. Check the current definitions before you assume either way.

Where a fire statement is required it must be prepared by someone with the necessary fire engineering competence under the Building Safety Act 2022 framework, and an AI draft is a starting point for that professional rather than a substitute. Engage the fire engineer before the layout is fixed, because escape strategy, stair capacity and the position of the rising main all change when storeys are added to an existing block.

Structural feasibility and the things outside planning

Prior approval says nothing about whether the building can carry the load. Establish that first, because the answer changes the construction method and therefore the design.

The usual sequence is an intrusive survey of the existing frame and foundations, a structural appraisal of the additional load, and then a decision between lightweight modular construction, a transfer structure or an independent frame. Each has different implications for the elevations, which is precisely what the prior approval will assess.

Title is the other early check. Airspace above a block of flats is frequently not owned by the person who assumes they own it, long leases may demise the roof, and the Landlord and Tenant Act 1987 gives qualifying tenants rights that can complicate a disposal of airspace. Resolve ownership before spending money on drawings.

Community Infrastructure Levy is payable on new dwellings created under permitted development where a charging schedule is in force, through a notice of chargeable development. Budget for it. Our note on Class B roof enlargement volume allowances covers the householder equivalent, where the arithmetic works quite differently.

Practical order of work

1. Identify the correct class from the current text of Part 20 and confirm the building type.
2. Evidence the construction date from documentary sources.
3. Confirm the land is not excluded, including a check for listing and conservation area status.
4. Carry out a measured survey and establish the height datum and existing storey heights.
5. Take structural and title advice before design work starts.
6. Test daylight to every proposed habitable room.
7. Prepare the prior approval application with a design statement, construction management statement and fire safety material.

Development under Part 20 must be completed within three years of the date of prior approval, so the programme is part of the feasibility rather than an afterthought. A well drafted planning statement accompanying the application should walk the officer through each condition in order and show it is met.

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

You can draft the planning statement and supporting documents for a prior approval application from the full report index. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.