Which reports an upward extension to a block of flats needs

Adding storeys to a block of flats is one of the few things the permitted development regime allows at real scale, and it is also one of the most heavily conditioned. Part 20 of Schedule 2 to the GPDO 2015 contains a set of classes permitting new dwellings on top of existing buildings, including a class for detached blocks of flats. The route is prior approval rather than full permission, but the list of matters the council may consider is long enough that the evidence bundle looks much like a planning application anyway.

Part 20 has been added to over time and the class letters differ by building type, so confirm which class applies to your building against the current consolidated text before committing to the route.

The prior approval route and its limits

The conditions that recur across the Part 20 upward extension classes, and that decide whether the route is available at all:

• A limit on the number of additional storeys, typically two where the existing building already has a stated number of storeys.
• A maximum height for the extended building, and a maximum height for each new storey measured floor to ceiling.
• A date range within which the existing building must have been constructed.
• Exclusion on Article 2(3) land, on sites of special scientific interest, safety hazard areas and military explosives storage areas.
• Exclusion where the building is listed or lies within the curtilage of a listed building or a scheduled monument.
• A requirement that the new dwellings are flats rather than houses, and that the works are engineering operations reasonably necessary to construct them.

Read the dimensional limits carefully against a measured survey rather than against the original architect's drawings. A parapet, a plant enclosure or a lift overrun can put an extended building over a height limit that the floor plates suggest it clears.

The prior approval matters are wider than on most classes. They have included transport and highways, air traffic and defence assets, contamination, flood risk, external appearance, the provision of adequate natural light in all habitable rooms, the impact on the amenity of the existing building and neighbouring premises including overlooking, privacy and loss of light, and, in some cases, protected views. Part 20 also requires a report dealing with the management of construction, covering hours of operation and how noise, dust, vibration and traffic effects on occupiers and neighbours will be mitigated.

Determination periods for Part 20 are not the 56 days familiar from other prior approval routes. Check the relevant paragraph of Part 20 and diary the actual date.

Structural capacity evidence

Nothing else matters if the building will not take the load.

What an engineer needs to establish, and what the submission should summarise: the existing frame or loadbearing structure and its residual capacity, the foundation type and bearing capacity against the increased load, lateral stability and how wind loading on a taller building is resolved, the condition of the existing roof structure and whether it can be removed while the building is occupied, and whether the existing stair and lift cores can serve additional floors or need extending.

Intrusive investigation is usually unavoidable: opening up to confirm reinforcement, concrete testing, trial pits at foundations. In an occupied block that needs access agreements with leaseholders, which takes time.

Lightweight construction, typically light gauge steel or cross-laminated timber, is the usual answer to a capacity constraint, and it changes the acoustic and fire design rather than removing the problem. Set the structural survey work in train before spending on architecture, because the outcome may cap the number of storeys at one rather than two.

Daylight and sunlight to existing flats

Building on top of a block reduces daylight to the flats immediately below at the perimeter, and can affect neighbouring buildings where the block is close to a boundary.

Assessment follows BRE 209, using vertical sky component and no sky line for daylight and annual and winter probable sunlight hours for sunlight, with the existing condition as the baseline. The usual finding on an upward extension is that the top floor flats of the host building lose the most, particularly where balconies or recessed windows already restrict sky visibility.

Two practical points. Run the daylight and sunlight assessment at concept stage, because the results drive setbacks at the top storey and the position of any roof plant. And report the results for the host building's existing residents openly. Prior approval expressly covers impact on the amenity of the existing building, so a study that assesses only neighbouring properties has answered half the question.

Where the results are marginal, a set-back top storey with a recessed perimeter often resolves both the daylight numbers and the external appearance objection at the same time.

Construction in an occupied building

This is the practical issue residents care about most, and Part 20 requires it to be addressed directly.

The construction management report should cover the working hours proposed, the route and timing of deliveries, where the crane or hoist stands and what it oversails, how the roof is opened up and made weathertight in stages, dust and noise control including the effect of removing an existing roof over occupied flats, how water ingress risk is managed, how residents keep access to entrances, parking and bin stores, temporary arrangements for services, and the complaints route with a named contact.

Phasing matters more than mitigation. A scheme that removes a section of roof at a time, working over a decanted floor, causes a fraction of the disruption of one that strips the whole roof in a week. Say which you are doing.

A construction management plan prepared to the standard a council would expect on a major application is proportionate here, even though the route is prior approval.

Fire safety and the height threshold

Adding storeys can push a building past 18 metres in height or seven storeys, which changes the regulatory position entirely.

Above that threshold a building containing two or more residential units falls within the higher-risk regime under the Building Safety Act 2022. At planning stage that engages Planning Gateway One, which requires a fire statement under Article 9A of the Development Management Procedure Order 2015 for relevant buildings, with the Building Safety Regulator as a statutory consultee. Gateways Two and Three follow at building control and completion.

Measure the proposed height properly and early. A scheme that lands at 18.4 metres carries a materially different programme, cost and consultant team from one at 17.6 metres, and the decision to lose a storey costs far less before the design is fixed.

A fire statement for a relevant building must be prepared by someone with the necessary fire-engineering competence, and that is a legal requirement rather than a recommendation. An AI-assisted draft is a starting point for that professional, never a substitute. The same applies to the fire safety strategy that follows under the Building Regulations.

Even below the threshold, adding storeys raises means of escape, stair capacity, fire service access and external wall construction questions that need a fire engineer's input.

An illustrative example

A five-storey purpose-built block of 20 flats, detached, built in the 1970s, with a flat roof and a single stair core. The engineer confirms the frame and foundations can carry one additional storey in light gauge steel but not two. The extended building measures 17.2 metres to the top of the new parapet, staying below the higher-risk threshold, though roof-mounted plant has to be relocated to keep it there. The daylight study shows two fourth-floor flats falling below the BRE reference values, so the new storey is set back 1.8 metres on that elevation. Construction is phased in three sections with a temporary weatherproof deck. This example is illustrative.

When a full application becomes the better route

Prior approval is attractive because the authority's discretion is limited. It stops being attractive in four situations.

Where the building fails one of the conditions, on height, construction date, designation or building type. Where you want more than the class allows, whether that is three storeys, a different footprint or a change to the ground floor. Where the site would benefit from a wider negotiation, for example trading additional units for a contribution or an improved entrance and street frontage. And where the daylight or amenity findings are bad enough that a refusal of prior approval looks likely, in which case a full application at least lets you argue planning balance.

A full application also produces a permission that cannot later be attacked on the basis that a condition of the class was never satisfied, which matters on sale and on refinance.

Work out which of the supporting documents your scheme needs before drawing anything, because the structural, daylight and fire work sets the envelope rather than responding to it. PlanScope provides AI-assisted draft reports for planning applications, and for specialist technical reports qualified professional input may be required. Reports should be reviewed before submission. We do not guarantee planning approval.

This is general guidance, not legal advice. Check the current text of Part 20 and the position with your local planning authority before relying on anything here.