Part 20 upward extensions: new flats on top of a block

Part 20 of Schedule 2 to the General Permitted Development Order 2015 allows new flats to be built on top of an existing block without a planning application. Class A of that Part covers the case most developers ask about: a detached purpose-built block of flats, gaining up to two additional storeys through prior approval rather than through the ordinary application process. Separate classes in the same Part deal with detached and terrace buildings in commercial or mixed use and with buildings already in use as dwellinghouses.

The right is real and it is used. It is also narrow, heavily conditioned, and it drops you into building safety territory the moment the extended building passes 18 metres.

Which blocks qualify

Start with the building itself.

It has to be a detached block of flats. A block attached to another building, or forming part of a terrace, falls outside this class and needs to be checked against the others in Part 20.

The existing building has to have enough storeys to begin with. The class is aimed at blocks that already read as a multi-storey building, so a two storey block will not qualify for the same allowance as a taller one. Count the storeys above ground level carefully, because a semi-basement or an undercroft car park can change the answer.

Construction date is the next filter. The right applies to buildings constructed within a defined window, so a block completed before the post-war cut-off or after the more recent one is excluded. Check the exact dates in the current text of the Order rather than relying on a figure from memory, because these dates have been the subject of amendment and they are absolute.

Designations remove the right entirely. Article 2(3) land, meaning conservation areas, National Parks, the Broads, National Landscapes and World Heritage Sites, takes the building out, as do listed status, curtilage of a listed building, scheduled monument status, sites of special scientific interest and safeguarded land around aerodromes and defence assets. Our guide to article 2(3) land covers those designations in more detail. An Article 4 direction can close the right locally as well.

Storey and height limits

Three limits operate together and the binding one varies.

The number of additional storeys is capped, at up to two for a qualifying block. The overall height of the extended building is capped in absolute terms, which becomes the governing limit on anything already tall. And each new storey is limited in internal floor to ceiling height, measured against a fixed figure or the height of an existing storey, whichever is lower.

Survey the existing building properly before testing the limits. Roof-level plant, lift overruns and parapets complicate the measurement, and an assumption about the existing ridge height that turns out to be 400 mm optimistic will invalidate the whole prior approval.

The prior approval matters

Part 20 carries a far longer list of prior approval matters than the householder right in Part 1, and each is a ground on which the council can refuse. Expect to address:

• transport and highways impacts, including parking and servicing
• air traffic and defence asset safeguarding
• contamination risk on the site
• flooding risk
• external appearance of the building
• provision of adequate natural light in all habitable rooms of the new flats
• impact on the amenity of the existing building and of neighbouring premises, including overlooking, privacy and loss of light
• protected views
• whether the development would affect an existing use on the site

Natural light is the one that catches developers out. It is a stated matter, which means a design with deep plan flats and single aspect bedrooms serving light wells will be assessed on it. Model it properly rather than asserting compliance.

External appearance is the other common refusal ground. A two storey addition in a different idiom on a 1960s block reads badly, and officers say so.

The council must also be given a report on the management of the construction works, covering hours of operation, dust, noise and vehicle movements, because the residents below will be living in the building throughout.

Structural and fire evidence

The application needs a structural report addressing whether the existing frame and foundations can take the additional load and what strengthening is proposed. Get that done before you commit, since many 1960s and 1970s blocks were designed with little spare capacity and the strengthening works can be the item that kills the appraisal.

Fire is the bigger issue. Where 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 acting as building control authority and gateways two and three applying. That is true whichever planning route you use.

Adding storeys to an occupied block also changes the fire strategy for the existing residents, not just the new ones. Evacuation strategy, the firefighting shaft, the rising main and the compartmentation at the interface between old and new all have to be resolved, and a Fire Safety Strategy is a fire-engineering document requiring a professional with the necessary competence. An AI-assisted draft is a starting point for that engineer, never a substitute.

Leaseholders, airspace and ownership

The planning route says nothing about whether you own what you propose to build in.

Check the freehold title and whether the airspace above the top floor was demised to the top floor leaseholders. Check the leases for covenants on alterations, on quiet enjoyment and on the use of common parts. Where a landlord proposes to dispose of an interest in the airspace, the right of first refusal owed to qualifying tenants under landlord and tenant legislation can apply, and getting that wrong is expensive.

Practical points that decide these schemes:

1. Establish airspace ownership before spending anything on design.
2. Check whether the leaseholders have exercised a right to manage or acquired the freehold collectively.
3. Work out how service charges will be apportioned once the block is larger.
4. Plan access and craneage, since the site is an occupied residential building with no compound.
5. Expect the residents to engage with the prior approval process, because the council will notify them.

Money and programme

Community Infrastructure Levy is payable on new dwellings created under permitted development where the council charges it. Because there is no planning permission to attach the liability to, the developer serves a notice of chargeable development before starting, and failing to do so removes the ability to claim any exemption or instalment arrangement. This is the single most common financial error on Part 20 schemes.

Affordable housing contributions do not arise, since there is no planning permission and no section 106. That is the main commercial reason the route exists.

On programme, allow eight weeks for the prior approval determination, and check the Order and the council's acknowledgement for what happens if that period passes without a decision, because the rules differ between permitted development classes. Then allow for building safety gateway two approval before work starts, which on a higher-risk building is a substantial period in its own right and is frequently underestimated.

The prior approval must be implemented within the period stated in the Order, so a scheme that sits on the shelf for years has to start again.

One honest uncertainty: the amount of structural strengthening needed is rarely known until the existing frame is investigated, and intrusive investigation in an occupied block is difficult to arrange. Price that risk rather than assuming the desktop appraisal holds.

Enter your postcode on PlanScope to see which reports an upward extension is likely to need, and read the Planning Statement page for what to cover if you take the full application route instead.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.