Part 20 upward extension of blocks of flats

Part 20 of the General Permitted Development Order allows up to two additional storeys of new flats to be built on top of an existing detached block of flats, through prior approval rather than a planning application. The building has to have been constructed between 1 July 1948 and 5 March 2018, the extended building must not exceed 30 metres in height, and each new storey is limited in floor to ceiling height.

The planning route is the straightforward part. What stops most airspace schemes is ownership, structure and the practicalities of building above people who already live there.

The rights sit in Schedule 2, Part 20 of the Town and Country Planning (General Permitted Development) (England) Order 2015 and have been amended since introduction, so read the current consolidated text before relying on any limit.

Which blocks qualify and the age requirement

The right applies to a purpose-built detached block of flats. Detached does the heavy lifting: a block joined to a neighbouring building, even by a link or a shared party wall, falls outside it.

The construction date has to fall between 1 July 1948 and 5 March 2018. Postwar blocks are the target, and the upper date excludes anything built recently, on the reasoning that a modern block was designed to its site capacity already.

The right is not available on land within a conservation area, a National Park, a National Landscape, a World Heritage Site or the Broads, nor on sites of special scientific interest, listed buildings, scheduled monuments or land in their curtilage. Safety hazard areas and military explosives storage areas are excluded as well. An Article 4 direction can remove the right locally, so check the council's register before spending money.

Storey and height limits

The new storeys must sit immediately above the topmost storey of the existing building, within its footprint. The Order permits engineering operations reasonably necessary to strengthen existing walls and foundations, which is essential given what the structural assessment usually finds.

The 30 metre cap is the one that removes most inner-urban towers from the scheme. Measure the existing building properly, from the datum the Order specifies, before assuming there is room for two storeys.

Prior approval matters including amenity and design

The prior approval list for Part 20 is long. It covers transport and highways impacts, air traffic and defence asset impacts, contamination risks, flooding risks, 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 of the new flats, the impact on the amenity of the existing building and of neighbouring premises including overlooking, privacy and loss of light, and, in relevant areas, protected views.

Two of those decide most applications.

Natural light is assessed for the new flats themselves, and roof-level layouts with deep plans and small windows fail it. Design the daylight in rather than defending it later.

Amenity of the existing building is the ground objectors use, and it is legitimate. Existing top-floor flats lose the sky above their windows, and neighbouring buildings can lose light to a taller mass. A Daylight and Sunlight Assessment using the BRE approach should test both the new flats and the existing and neighbouring ones, and it should be commissioned before the massing is fixed rather than in response to a consultation objection.

Fire safety and the 18 metre interaction

Adding two storeys frequently takes a building past 18 metres in height or seven storeys, which brings it within the higher-risk building regime under the Building Safety Act 2022. That changes the building control route entirely, since a higher-risk building goes through the Building Safety Regulator's gateway process rather than conventional building control, with a stop point before construction begins and another before occupation.

Check the resulting height and storey count at concept stage. A scheme that lands just over the threshold carries a materially longer programme and a different cost base from one that lands just under, and that difference is often larger than the value of the second storey.

Existing fire strategy matters too. Many 1960s and 1970s blocks have a single staircase and an evacuation strategy premised on the building as built. Adding storeys changes travel distances, escape capacity, firefighting access and the smoke control assumptions. Where a Fire Statement is required for the application, it must be prepared by someone with the necessary fire engineering competence under the Building Safety Act 2022 framework and Article 9A of the Development Management Procedure Order 2015, and an AI-assisted draft is a starting point for that professional rather than a substitute.

Construction impacts and existing residents

Building two storeys on an occupied block is a difficult construction job and the council will want to see how it is managed.

The issues are consistent: crane siting and oversailing, scaffolding fixed to an occupied building, protection of the existing roof during the works, noise and hours, loss of parking to site compound, service interruptions, waste and deliveries, and insurance. A Construction Management Plan that addresses each of these in specific terms is worth preparing for the prior approval application even where it is not strictly required, because it answers the question residents will raise at the earliest opportunity.

Programme matters to residents more than to anyone. Off-site manufactured modules reduce time on site substantially and are common on airspace schemes for exactly that reason, though they impose their own constraints on grid, tolerance and crane capacity.

Leasehold and structural feasibility realities

This is where schemes die, and it has nothing to do with planning.

Ownership of the airspace above the top floor is a question of title. Where flats are held on long leases, the demise may include parts of the roof or roof void, and the freeholder's ability to build above may be limited by the leases, by rights granted to leaseholders, or by covenants for quiet enjoyment. Where the freeholder proposes to dispose of an interest in the airspace, leaseholders may have statutory rights of first refusal, and getting that process wrong is expensive.

Structure is the second gate. A block designed in 1968 for its own weight rarely has spare capacity for two storeys without intervention. A Structural Survey with intrusive investigation of the frame, the slabs and the foundations is the first spend on any serious appraisal, and it should be done before design fees are committed. Large panel system blocks need particular care and specialist appraisal.

Then the services. Lifts that do not reach the new floors, risers with no spare capacity, water pressure at the top of a taller building, roof plant that has to be relocated, and drainage stacks that were never sized for more flats. Each is solvable and each costs money.

Where a planning application is the better route

Part 20 is attractive because the council cannot object in principle and cannot require affordable housing through the prior approval process. Those are real advantages.

They come with real constraints. The footprint is fixed, the height is capped, the design freedom is limited, and the prior approval matters are wide enough that a poor scheme will still be refused. Where the site has surrounding land, where a different massing would work better, or where the building is not detached or falls outside the date range, a full planning application gives you the site rather than the roof.

A hybrid approach is common. Establish whether the Part 20 scheme is deliverable, use it as a fallback in the planning balance, and apply for the better scheme with that fallback set out in the Planning Statement. The fallback argument only works where the alternative is in fact capable of being implemented, so the structural and ownership work has to be done either way.

Browse the 66 report types by category to build the evidence list for an airspace scheme before you commit to a route.

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.