Documents for adding storeys to a block of flats

Adding storeys to an existing block of flats brings together four evidence streams that rarely sit in the same application: structural capacity of the building you did not design, daylight to flats you do not own, fire strategy for a building that may cross the higher-risk threshold on completion, and construction logistics on a site where people are living throughout. The documents follow from those four, and the order you produce them in decides whether the scheme is feasible before you spend money on drawings.

Start with the structure. An upward extension that the existing frame and foundations cannot take is not a planning problem, and no amount of design work fixes it.

The prior approval route and its limits

Part 20 of Schedule 2 to the General Permitted Development Order 2015 allows up to two additional storeys of new dwellings above the topmost storey of a purpose-built detached block of flats, subject to conditions. The building must have at least three storeys above ground level and have been constructed within the period the class specifies, and the extended building is capped at 30 m in height.

The route is closed on a good deal of land. Conservation areas, National Parks, National Landscapes, the Broads and World Heritage Sites are excluded, along with sites of special scientific interest, listed buildings and their curtilage, scheduled monuments and safety hazard areas. A terraced block is also outside Class A, which applies to detached blocks, so check the class against the building form before assuming the route is available.

Prior approval under Part 20 is wide-ranging. The authority considers transport and highways impacts, air traffic and defence asset impacts, contamination, flooding, the external appearance of the building, natural light in all habitable rooms of the new dwellings, the amenity of the existing building and neighbours including overlooking and loss of light, protected views, and air quality. That list covers most of what a full application would consider, which is why the prior approval pack is rarely slim.

Where the building sits on excluded land, or where the design needs more than two storeys or breaks a condition of the class, a full application is the route and the document set is broadly the same.

Structural capacity and the survey it needs

The existing building's drawings, if they survive, are the starting point and they are often wrong. What settles the question is intrusive investigation: opening up to confirm slab and beam construction, foundation trial pits to establish depth and bearing, concrete testing where reinforcement details are unknown, and an assessment of the lift and stair cores.

Three capacity questions decide the scheme:

1. Can the foundations take the additional load, or is underpinning or load transfer needed?
2. Can the frame take it without strengthening that would require work inside occupied flats?
3. Does the existing lift and escape stair serve the additional storeys, and if not, where does new vertical circulation go?

That third question kills more upward extension schemes than the first two, because a new core has to land somewhere and the ground floor is usually full.

A Structural Survey is a report councils expect from a suitably qualified author, normally a chartered structural engineer, and professional review is strongly advised. Disproportionate collapse requirements also change with building height, so the structural strategy and the fire strategy have to be developed together rather than in sequence.

Daylight and sunlight to existing and neighbouring flats

Two sets of receptors matter. The flats in the building below, whose windows may be affected by the new storeys only slightly, and the neighbouring buildings, which can lose a good deal where the block sits close to a boundary.

The assessment follows the BRE guidance on site layout planning for daylight and sunlight, reporting vertical sky component and no sky line for daylight and annual probable sunlight hours for sunlight, with the retained values expressed as a proportion of the existing. Run it early, because the massing that works structurally may not work for the neighbours and the fix is usually a set-back upper storey.

The new dwellings themselves need adequate natural light in all habitable rooms, and that is a prior approval matter in its own right under Part 20. A top-floor flat with a single north-facing window into a deep plan will attract a refusal, so test the internal daylight provision as part of the layout rather than afterwards. A Daylight and Sunlight Assessment covers both directions of that question.

Fire safety on a taller building

Height changes the regime. Where the completed building reaches 18 m or seven storeys and contains two or more dwellings, it falls within the higher-risk category under the Building Safety Act 2022 framework, and the planning stage engages Gateway One with the Building Safety Regulator as consultee.

Article 9A of the Development Management Procedure Order 2015 requires a fire statement with applications for planning permission for development of a relevant building. Where the chosen route is prior approval rather than a full application, ask the authority how it expects the fire evidence to be presented, because practice varies and an unanswered fire question stalls the determination either way.

A Fire Statement is a report where a professional with the necessary fire-engineering competence is legally required, and an AI-assisted draft is a starting point for that professional rather than a substitute. Expect the strategy to deal with means of escape from the new storeys, the single stair question where one exists, fire service access and water supply, and the external wall construction of the addition.

Construction access, cranes and resident disruption

Part 20 requires a report on construction management, and on an occupied block it carries real weight. The authority wants to know working hours, how materials reach the roof, where the crane or hoist stands, how residents keep access to entrances and parking, and how noise, dust and vibration are controlled.

But crane positioning is the practical constraint. An oversailing jib across a neighbour's land needs their agreement, and the absence of that agreement has stopped schemes that had planning permission. Modular or panelised construction reduces the time on site and is worth describing where it is proposed.

A Construction Management Plan covering hours, deliveries, wheel washing, dust suppression and resident communication is usually conditioned, and submitting a draft with the application shortens the discharge later.

Amenity space, refuse and parking recalculated

Adding dwellings changes every per-unit calculation in the original consent. Refuse storage sized for 24 flats does not serve 32. Cycle parking provision has to grow. Amenity space, where the local plan requires it, has to be found on a site that was laid out without the extra homes in mind.

Parking is the usual argument. Where the block has an existing car park at a ratio the authority now considers too high, reallocating spaces to the new dwellings can work. Where parking is already short, a parking stress survey and a car-capped scheme with controlled parking zone evidence is the way the question gets answered.

The pack and the determination period

A realistic submission for an upward extension of a block of flats runs to a structural appraisal, a daylight and sunlight assessment covering existing flats, neighbours and the new dwellings, a fire strategy or fire statement, a construction management report, a transport and parking note with a stress survey where needed, air quality and noise assessments where the prior approval matters engage them, a refuse and cycle strategy, and the design drawings with existing and proposed elevations.

Prior approval under Part 20 runs to eight weeks. Where the application is incomplete the clock does not start, and incomplete here usually means the construction management report or the natural light evidence was missing.

Enter your postcode on PlanScope to see which of the 66 report types an upward extension is likely to need, with the full set listed at all 66 report types. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.