Adding storeys to a block of flats: the two routes

Upward extension planning permission for flats can come from two places: a prior approval under Part 20 of Schedule 2 to the General Permitted Development Order 2015, or a full planning application. For a purpose-built detached block of flats the relevant class is Class A of Part 20, which permits up to two additional storeys of new dwellings above the topmost residential storey. Classes AA to AD in the same Part deal with commercial, mixed use and single dwellinghouse buildings, so check which one your building actually falls into before anyone draws a roof plan.

The choice between the two routes is not obvious, and the cheaper one is often the slower one.

The permitted development route for additional storeys

Class A permits construction of new dwellings immediately above the existing topmost residential storey, together with the engineering operations needed to support them. That includes strengthening existing walls and foundations, plant, and works to provide access and services to the new dwellings.

Two additional storeys are permitted where the existing building already has three or more storeys above ground level. Where it has fewer, only one additional storey is permitted. That single sentence decides a great many schemes on two-storey blocks.

The right also allows works to the existing roof, plant enclosures and ancillary structures within limits set out in the class. Anything beyond them, including balconies to the new flats or an external stair, needs a full application.

Height limits, age of the building and excluded areas

The constraints are dimensional and absolute.

Excluded land removes the right entirely: conservation areas, National Parks, the Broads, National Landscapes and World Heritage Sites, sites of special scientific interest, land within the curtilage of a listed building, scheduled monuments, safety hazard areas and military explosives storage areas. Article 4 directions can remove it elsewhere, and several authorities have made them for exactly this purpose.

Note the 30 metre ceiling against the 18 metre threshold in the Building Safety Act 2022 regime. A block at 16 metres taking two storeys will cross 18 metres, and the scheme changes character when it does.

The prior approval matters the authority can consider

Prior approval is not a notification. The authority considers a defined list, and for Class A it is long: transport and highways, air traffic and defence assets, contamination, flood risk, the external appearance of the building including the design of the principal elevation and any side elevation fronting a highway, air quality for the new occupiers where existing uses nearby affect it, the amenity of the existing building and neighbouring premises including overlooking, privacy and loss of light, protected views where they apply, whether all habitable rooms will have adequate natural light, and the impact of construction.

The construction point deserves attention. The developer has to provide a report for the management of the construction, covering hours of operation and how dust and noise will be controlled. On an occupied block that report is the document residents read most closely, and a thin one invites objections that the authority can act on through this very heading.

Later amendments added requirements that new dwellings meet the nationally described space standard and that habitable rooms receive adequate natural light. Both are checked at prior approval stage and both are common reasons for refusal.

Daylight and sunlight to existing occupiers

Two separate questions arise and applicants regularly answer only one.

The first is daylight and sunlight to neighbouring buildings, assessed using the BRE guidance on site layout planning for daylight and sunlight. On a block surrounded by open space and parking, adding two storeys often has little effect, and the assessment is short.

The second is daylight to the existing flats in the same building, which is where upward extensions hurt. New plant, a lift overrun, a taller stair core or an extended flank wall can shade the top floor flats that were previously unshaded. Balcony additions and screening to the new flats can reduce light to the windows beneath them. A daylight and sunlight assessment has to cover both, and the existing occupiers are the ones who will write to the council.

Internal light in the new flats is the third strand. A single aspect flat under a deep roof overhang, with the only window facing a parapet, will not pass the natural light test.

Structural capacity and the survey evidence

Adding two storeys loads foundations designed in the 1960s for the building that exists. The structural question comes before the planning question, because a scheme that cannot be built without underpinning half the block is not a scheme.

What the evidence needs to establish:

1. The original construction: large panel system, cross-wall masonry, in situ frame or steel, from the original drawings where they survive.
2. Foundation type and depth, confirmed by trial pits rather than assumed.
3. Ground conditions and bearing capacity, from a ground investigation on the site itself.
4. The spare capacity in walls, columns and foundations, and the strengthening needed to take the additional load.
5. Whether large panel system construction is present, which brings its own assessment requirements and often ends the scheme.
6. Disproportionate collapse requirements at the new building height.

A structural survey is the starting document, and a structural engineer's appraisal with a load path and a strengthening strategy is what makes the appraisal bankable. Lightweight construction for the new storeys, usually steel or cross-laminated timber, is normal for a reason.

Fire safety, means of escape and Gateway One

If the finished building is at least 18 metres tall or has at least seven storeys and contains dwellings, it falls within the higher-risk building definition, and the planning application or prior approval must be accompanied by a fire statement and the Building Safety Regulator becomes a statutory consultee at Planning Gateway One.

A Fire Statement at Gateway One has to be prepared by someone with the necessary fire engineering competence. This is not a document to draft and submit unreviewed: a competent professional is required, and an AI-assisted draft is a starting point for that person rather than a substitute for their judgement.

Beyond the statement, the practical issues are the ones that decide whether the scheme works: a single existing staircase serving more storeys, the fire service access and water supply at the new height, the evacuation strategy for the existing residents during construction, and the fire resistance of the new floor over the existing top flats. Where the existing block has one stair and no lift, adding two storeys of flats is a hard case.

The rooftop plant that comes with the new flats brings its own planning questions, set out in rooftop plant enclosures.

Choosing between prior approval and a full application

Prior approval looks quicker and often is not. The limits are rigid, the natural light and space standard tests are unforgiving, the design of the elevations is assessed anyway, and there is no scope to negotiate a slightly different scheme in return for a better one.

A full application gives you flexibility on footprint, balconies, external stairs, a larger uplift in units and a scheme that can be amended during determination. It costs more in fees and evidence, it may attract affordable housing and other obligations, and it is decided against the local plan rather than a closed list.

The honest test is whether the building sits comfortably inside every Class A limit with a scheme you actually want to build. If it does, use the prior approval. If you find yourself designing to the limits rather than to the site, apply for planning permission. Ownership of the airspace, freeholder consent, leaseholder rights and party wall matters sit outside the planning system entirely and need legal advice early, because a consent you cannot implement is worth nothing.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it, and read the current text of Part 20, which has been amended several times. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval, and for fire and structural documents a competent professional is legally required.

Browse the 66 report types to see which documents an upward extension scheme is likely to need.