Part 20 upward extensions to create new dwellings

Part 20 of Schedule 2 to the GPDO 2015 allows new flats to be built on top of existing buildings by prior approval rather than by planning application, within fixed storey and height limits and subject to a construction date condition that rules out a large share of the stock. The classes cover blocks of flats, buildings in commercial or mixed use and houses, with up to two additional storeys where the existing building already has enough storeys, an overall height cap, and a long list of prior approval matters the authority can refuse on. It is a real route to new homes. It is also the least forgiving part of the Order to get wrong, because a defective prior approval leaves you building without permission.

Read the class text itself before doing anything else. The classes were added in 2020 and amended since, and secondary summaries drift.

Which buildings the Part 20 classes cover

Part 20 contains several classes with different subject buildings. Class A deals with new dwellings on detached blocks of flats. Further classes cover new dwellings on buildings in commercial or mixed use, both detached and in a terrace, and on buildings in use as dwellinghouses, again both detached and in a terrace. Class ZA is different in kind: it permits demolition of a building and construction of new dwellings in its place.

Identifying the right class is the first task and it is not always obvious. Whether a building is detached or part of a terrace, and whether its existing use is residential, commercial or mixed, changes which class applies, which height cap applies and which construction date applies.

Two general points apply across the Part. The new units must be flats, and the development must create dwellings in Class C3 use. And the existing building must be capable of taking the load, which is a structural question the Order says nothing about but which the lender and the building control body will ask about first.

Storey and height limits

The pattern is consistent even though the figures differ by class. Where the existing building has enough storeys, up to two additional storeys are permitted. Where it has fewer, one additional storey.

Measure from the right datum and to the right point. The Order defines how height is measured for these classes, and a parapet, a plant enclosure or a lift overrun can take a scheme over a cap that the occupied floors sit comfortably below.

Where the extended building reaches 18 m or seven storeys it falls within the higher-risk building definition under the Building Safety Act 2022 regime for building control purposes, which changes the construction route entirely. Confirm with the authority whether a fire statement is expected for the planning route, and get fire engineering input on the core and escape strategy before the layout is fixed.

The construction date conditions

Every upward extension class carries a condition about when the existing building was constructed, and this is where most sites fail.

The earliest date is 1 July 1948 across the classes. The later cut-off is either 5 March 2018 or 28 October 2018 depending on the class, so check which applies to the class you are relying on rather than assuming. A building completed after the relevant cut-off is outside the class however suitable it looks.

Evidence of the construction date is your problem, not the authority's. Useful sources are the original planning permission and the building control completion certificate, historic Ordnance Survey mapping, the title register and historic aerial photography. Assemble it before submission, because a prior approval granted on an unevidenced date is a weak foundation for a development.

Prior approval matters including amenity and external appearance

A prior approval application is not a notification. The authority considers a defined list of matters and can refuse on any of them. Across the upward extension classes those matters include:

• Transport and highways impacts of the development
• Air traffic and defence asset impacts
• Contamination risks and flooding risks on the site
• The external appearance of the building, including the design and architectural features of the principal elevation and any side elevation fronting a highway
• The provision of adequate natural light in all habitable rooms of the new dwellings
• Impact on the amenity of the existing building and neighbouring premises, including overlooking, privacy and loss of light
• Air quality impacts on the intended occupiers
• Impact on protected views where the site is affected by a relevant direction

External appearance is the most commonly fatal. Two storeys added to a 1960s block in a street of two storey houses is an appearance question the authority is entitled to refuse on, and a design that reads as part of the host building rather than as a visible addition is the only answer.

Natural light is the second. The requirement for adequate natural light in all habitable rooms applies, and dwellings created under permitted development must also meet the nationally described space standard. A unit with a habitable room lit only by a rooflight over a deep plan needs testing with a daylight and sunlight assessment before submission rather than after refusal.

Protected views, air traffic and transport matters

Three matters sit outside the designer's control and need checking at the feasibility stage.

Protected views are the London issue. Where a direction protects a vista, a scheme that intrudes into it will not pass, and the check takes an afternoon.

Air traffic and defence assets matter more widely than people expect. Proximity to an aerodrome, a radar installation or a safeguarded defence site brings consultees whose objections are difficult to overcome, and the safeguarding maps are the place to start.

Transport and highways impacts usually turn on parking and servicing in an existing street with no capacity. Where the authority operates a controlled parking zone, expect a permit free condition or an obligation to be discussed, and prepare the evidence rather than arguing the point.

Where the classes are excluded

The classes do not apply at all on certain land. Article 2(3) land is excluded, so conservation areas, National Parks, National Landscapes, the Broads and World Heritage Sites are out. Listed buildings and land within their curtilage are out. Scheduled monuments and land within their curtilage are out. Sites of special scientific interest, safety hazard areas and military explosives storage areas are excluded as well.

Check the full exclusion list in the class text, because it is longer than the headline designations and includes some categories that rarely come up until they do. An Article 4 direction can also remove the rights, which is the check people forget because Article 4 directions are usually associated with householder works and office conversions, as covered in Class MA vacancy and continuous use conditions.

Fees and the information to submit

A fee is payable per new dwelling under the current fee regulations, and planning fees are indexed annually from 1 April, so check the current schedule rather than a figure from last year.

The submission needs the information the class requires, typically a plan showing the site, the existing and proposed elevations and floor plans, and a written description of the development. Add the construction date evidence, the natural light assessment, a structural appraisal and a draft construction management plan, because the classes impose conditions about construction hours and the management of construction that the authority will want addressed.

One point that catches developers out: new floorspace created under permitted development can still be liable for the community infrastructure levy, and where a prior approval is required the liability is captured by submitting a notice of chargeable development. Miss it and the surcharge follows.

The determination period is eight weeks, and the consequences of the authority not notifying a decision in time are set out in the conditions attached to each class. Read them, because proceeding on an assumption about deemed approval is an expensive way to find out.

Browse the report types to see which documents an upward extension scheme is likely to need.

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.