Part 20 prior approval or a full application upwards

Choosing between Part 20 prior approval or a full application for an upward extension comes down to one question: does the building satisfy every condition in the relevant Part 20 class, including the construction date window and the height caps? If it does, prior approval is faster and the matters the council can consider are limited by the Order. If any condition fails, there is no half-way route. The scheme becomes a planning application assessed against the whole development plan, and the design is free to improve as a result.

Read the conditions before you commission drawings. That order of work saves the most money.

What the upward extension classes permit

Part 20 of Schedule 2 to the General Permitted Development Order 2015 contains several separate classes, and people routinely argue about the wrong one. Class A allows new flats on a detached block of flats. Classes AA and AB allow new dwellings above detached and terrace buildings in commercial or mixed use. Classes AC and AD allow new dwellings above terrace and detached houses. Class ZA is different in kind: demolition of a single building and replacement with a new block of flats or a house.

Each class grants a different number of storeys and a different height ceiling, and each has its own list of works that may be carried out at the same time, such as access, services, plant and storage. Quoting the limits from one class while applying for another is the most common reason a prior approval application is refused on its own terms.

Storey limits, height limits and the age cut-off

Three gates close most buildings out.

The construction date is the first. The classes that add storeys apply to buildings constructed between 1 July 1948 and 5 March 2018. A Victorian terrace is out. A block finished in 2019 is out. Evidence of the date of construction is part of the application, and historic mapping or the original building control record is what satisfies it.

The storey count is the second. A building with two or more existing storeys can generally take up to two additional storeys, while a single storey building can take one. The new storeys sit on the principal part of the building, not on an outrigger or a later extension.

The absolute height is the third, and it is measured on the extended building rather than on the addition. There is also a limit on the floor to ceiling height of each new storey, which is set by reference to the existing storeys, so a building with tight floor to ceiling heights cannot be topped with generous ones.

Prior approval matters the council may consider

The list is set out in each class and it is longer than most applicants expect. Transport and highways impact. Contamination and flood risk. The external appearance of the building. Air traffic and defence asset safeguarding. The provision of adequate natural light in all habitable rooms of the new dwellings. The effect on the amenity of the existing building and of neighbouring premises, including overlooking, privacy and loss of light. Impact of noise from any commercial premises in the building. For some classes, the effect on a protected view.

Two of those decide most cases. Natural light is a pass or fail matter rather than a balancing exercise, so a new top floor flat with a single aspect onto a light well is vulnerable. Amenity of neighbours covers loss of light to existing windows, which means a Daylight and Sunlight Assessment is often needed even though this is a permitted development route.

The application must also be accompanied by a structural engineering report showing that the existing building can take the additional storeys, and by details of how construction will be managed, including hours of operation and how vehicles will access the site. New dwellings created through permitted development must meet the nationally described space standard.

Designations that exclude the right

Part 20 does not apply on land described in Article 2(3) of the Order, which covers conservation areas, National Landscapes, National Parks, the Broads and World Heritage Sites. It does not apply to a listed building or land within its curtilage, nor to a scheduled monument or its curtilage, nor to a site of special scientific interest, a safety hazard area or a military explosives storage area.

Check these first, because they are binary. One conservation area boundary running through the site ends the conversation. The datasets behind most of these designations are published on planning.data.gov.uk, and the authority's own constraint layer should be checked against them.

Structural and fire evidence needed on either route

The engineering work is the same whichever route you take. An upward extension loads an existing frame and foundation that were designed for something else, so a Structural Survey and then a full structural appraisal are unavoidable. Lightweight construction, often a steel or cross-laminated frame, is usually what makes the numbers work.

Fire is where the routes diverge on paperwork. A full application for development involving a relevant building, broadly one of at least 18 metres or seven storeys containing two or more dwellings, must be accompanied by a Fire Statement under Article 9A of the Development Management Procedure Order 2015. The prior approval route is not an application for planning permission, so confirm with the authority what it expects to see. Either way the Building Safety Regulator's gateways apply to the construction itself, and an extended building that crosses the height or storey threshold becomes a higher-risk building with all that follows.

Also check the single staircase question and the means of escape from the new storeys early. A scheme that cannot evacuate is not a scheme, whatever the planning route says.

Leasehold and party structure obstacles

Permitted development rights grant nothing in private law. In a block of flats the airspace above the roof is usually retained by the freeholder, so the developer needs either to own the freehold or to acquire an airspace lease. Where the freehold is owned by a residents' company, expect a negotiation rather than a transaction.

Statutory rights can sit in the way. A disposal of an interest in the building may trigger the leaseholders' right of first refusal, and the lease itself may contain covenants on alterations, roof access and quiet enjoyment. On a terrace, the Party Wall etc. Act 1996 applies to work on the party structure and needs notices served before work starts. None of this is planning, and all of it can stop a consented scheme dead. The distinction between a restrictive covenant and a planning condition is worth understanding before exchange.

When a full application produces a better scheme

Prior approval looks attractive because the matters for consideration are limited. The limits cut the other way too. You cannot exceed the height cap by half a metre to get a decent ceiling. You cannot set the new storeys back to improve the roofline if that leaves the floor plate unusable. You cannot add a storey to the back addition. And the external appearance test gives officers a way to object to the thing the limits force you to design.

A full application lets the scheme respond to the street, trade height against setback, and include a well-designed top floor with proper outdoor space. It carries affordable housing and policy obligations at the relevant thresholds, and it takes longer on a major scheme. On a constrained urban block, that trade is often worth making, and some applicants run both in parallel, using the prior approval as a fallback.

An illustrative example. A four storey 1960s block of flats, detached, in a suburban street outside any designation, with housing opposite at three storeys. Two additional storeys under Class A sit within the height cap, but the natural light test fails for two of the proposed flats whose only windows face a narrow flank. The full application version drops to one additional storey with a larger floor plate and dual aspect units, and reads better from the street. That example is illustrative only.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. To see the document pack each route needs, browse the PlanScope report types by category and start a draft.

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