Reports needed to add storeys to an existing block

Reports for an upward extension start with a structural question, not a planning one. Before anything else, someone competent has to say whether the existing frame, foundations and lateral stability can take the load. Part 20 of the General Permitted Development Order 2015, which contains the prior approval routes for additional storeys, expects that evidence with the application, and a full planning application will not get far without it either.

After that, the pack is shaped by two things: who already lives in the building, and how tall it becomes. Existing occupiers bring daylight, amenity and construction impact into play in a way a greenfield scheme never does. Height brings fire safety, and above a threshold it brings a statutory consultee with it.

Prior approval routes and their limits

Part 20 contains several classes permitting new dwellings to be built on top of existing buildings: blocks of flats, buildings in commercial or mixed use, terraces and detached houses each have their own class. Each carries its own conditions, and they differ.

The common features are a window of construction dates for the existing building, a cap on the number of additional storeys, a cap on the resulting overall height, a limit on the added floor to ceiling height, and a list of excluded land such as conservation areas, National Parks, listed buildings and their curtilage, and scheduled monuments.

Read the class that fits your building rather than a summary of Part 20 as a whole. The classes have been amended since they were introduced, and the difference between a detached block and a terrace determines which applies.

The prior approval matters are wider than in most parts of the Order. They typically include transport and highways, air traffic and defence assets, contamination, flooding, the external appearance of the building, provision of adequate natural light in all habitable rooms of the new and existing dwellings, the impact on the amenity of the existing building and neighbouring premises including overlooking, privacy and loss of light, and the impact of construction.

Structural capacity of the existing building

The structural work divides into assessment and design. Assessment establishes what is there: the frame type, the floor construction, the foundation form and depth, the lateral stability system and the condition of the fabric. Design establishes whether the proposed load can be carried and what strengthening is needed.

Start with the record drawings if they exist, and assume they do not. Most buildings of the relevant age have incomplete documentation, so intrusive investigation is usually necessary: opening up to confirm slab and beam construction, trial pits to establish foundation depth and bearing, and material testing on concrete or masonry.

The output is a Structural Survey and an engineer's report confirming whether the building can support the additional storeys, with the strengthening measures identified. Councils expect a suitably qualified author, and for a prior approval application the class itself sets out what must accompany the submission, so read that list before commissioning.

Two findings commonly change the scheme. Foundations that cannot take the load push the design towards a lightweight structure, usually a steel or timber frame with a lightweight envelope. Lateral stability that depends on cross walls limits where openings can go in the new floors.

Do this before the architect fixes a layout. Reworking a consented roof level because the engineer needs a new column line is an expensive sequence.

Daylight, sunlight and amenity for existing occupiers

Adding storeys reduces daylight to the flats below, to the neighbouring buildings and to any amenity space. That is the single most common source of objection, and it comes from people who already live in the building.

A Daylight and Sunlight Assessment should assess the effect on existing windows using the recognised site layout methodology, room by room, and report the results against the relevant targets. Where the building has balconies or deep reveals, model them, because they change the result substantially.

Assess the new dwellings as well. Adequate natural light in all habitable rooms is a prior approval matter under Part 20, and a top floor flat behind a parapet or set back behind a terrace can fail it.

Overlooking is the other half. New windows at roof level look into gardens and windows that were previously private, so draw the sightlines, consider set backs from the parapet line, and use obscure glazing or screening where a direct relationship cannot be designed out.

Communal amenity space matters too. Where the scheme adds units without adding amenity space, the per unit provision falls, and local plan policy usually has something to say about it.

Fire safety and the consequences of added height

Height changes the regime. Where a building reaches 18 metres or seven storeys and contains two or more residential units, it falls within the higher risk category under the Building Safety Act 2022 framework, and a planning application for such a building engages Planning Gateway One under Article 9A of the Development Management Procedure Order 2015.

That means a Fire Statement submitted on the prescribed form, and the Health and Safety Executive as a statutory consultee on the planning application. The statement has to be prepared by someone with the necessary fire engineering competence. This is one of the report types where a competent professional is legally required, and an AI assisted draft is a starting point for that professional rather than a substitute for them.

An upward extension is exactly the kind of scheme that crosses the threshold without anyone noticing. A six storey block at 17 metres becomes an eight storey block at 22 metres, and the whole fire strategy changes: means of escape, the single stair position, firefighting access, dry or wet rising mains, and the external wall construction.

Check the resulting height early, measured the way the regulations measure it, and get a fire engineer involved before the storey count is fixed.

Servicing, lifts and refuse capacity

An existing building was serviced for the number of flats it had. Adding more tests every one of those systems.

Work through the list with the building's managing agent. Lift capacity and whether the shaft can be extended. The foul drainage stack and whether it can take the additional connections. Incoming water pressure and whether a booster set is needed. Electrical supply capacity and the distribution network operator's position. Refuse storage volume against the collection authority's figures, and whether the existing store can be extended. Cycle parking against the local standard. Car parking, and whether the scheme is proposed as car free.

Two of these routinely become planning conditions. Refuse capacity, because the waste officer comments on it, and cycle storage, because it is a local plan requirement with a number attached.

The construction logistics deserve their own document. Building over an occupied block means a crane or hoist position, a scaffold design that does not block escape routes, working hours, noise and dust control, and a resident liaison arrangement. Submit a construction management plan with the application rather than leaving it to a pre commencement condition.

External appearance and townscape evidence

On the prior approval route, external appearance is a matter the authority can consider, so it is not a free pass. On a full application it is assessed in the round.

The evidence that works is visual. Verified views from agreed positions, a street elevation showing the building in its terrace or group, roofscape studies from the surrounding streets, and a materials strategy that relates the new floors to the existing building without pretending to be original.

Set backs are the usual design answer, and they cost floor area. A top floor set back from the parapet reduces the visual impact from the street and improves the daylight result for the neighbours, so the trade is worth modelling early rather than conceding late.

Where the building sits near a designated heritage asset or a protected view, add the heritage and townscape analysis that the specific constraint calls for.

Where a full application is the better route

Choose the full application where the building is on excluded land, where it falls outside the construction date window, where you want more storeys than the class permits, where the design needs changes the class does not cover, or where the scheme also involves alterations at ground level such as a new entrance, a substation or a refuse store.

The full route also lets you deal with the whole building at once, which is often the commercially better answer: refurbishment, re cladding, a new entrance and the additional storeys in one permission rather than a prior approval plus two further applications.

Where the addition takes the scheme to ten or more dwellings, further requirements come into play, as set out in our note on what changes in the report pack at ten dwellings.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. Enter your postcode on PlanScope to see which of the 66 report types an upward extension is likely to need. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.