Structural evidence for adding two storeys to a block

Upward extension structural evidence has to answer one question before anything else: can the existing frame and foundations carry two more storeys without works that are impossible to deliver while people live in the building. Everything else follows from that. Class AA of Part 20 of Schedule 2 to the General Permitted Development Order 2015 permits up to two additional storeys of new homes above the topmost residential storey of a purpose-built detached block of flats, within height caps, but the class says comparatively little about structure. The structural case is therefore mostly a Building Regulations and feasibility exercise that has to be done before you rely on the planning route at all.

What the structural case has to establish

Four propositions, each of which needs evidence rather than assertion.

The vertical load path can take the additional load, from the new floors through columns or walls to the foundations, with the existing members checked against current design codes rather than the code they were designed to.

Lateral stability is adequate at the new height, which is a different calculation and often the binding one.

The works can be built above an occupied building, with a temporary works strategy that keeps the existing flats habitable or sets out a decant.

Disproportionate collapse requirements are satisfied for the building class the finished structure falls into, which may change when storeys are added.

Write these as four headed sections in the report. An engineer's letter saying the building is suitable for two additional storeys in principle is worth nothing to a planning officer, a building control body or a funder.

Existing frame and foundation capacity

The first task is finding out what is actually there. Original drawings, if they exist, are a starting point and not an as-built record.

Expect to need opening up: concrete cover and reinforcement scanning, core sampling for concrete strength, and in older buildings an assessment for high alumina cement, large panel system construction or non-standard precast frames, all of which change the answer sharply. Buildings put up in the 1960s and early 1970s are where most surprises live.

Foundations are harder, because they are buried under an occupied building. Trial pits at a sample of locations, with recorded founding depth and dimensions, plus ground investigation to establish bearing capacity and settlement parameters, are the minimum. Where the foundation capacity is marginal, the options are load reduction through lightweight construction, load transfer to new elements, or underpinning, and underpinning beneath an occupied block is expensive and slow.

Soil parameters and the way they are derived belong in a geotechnical assessment, and the article on ground parameters inside a geotechnical assessment sets out what that document should contain.

Lateral stability with added height

Adding two storeys increases wind loading on the structure and raises the centre of pressure, so overturning moments and shear at ground level rise by more than the proportional increase in height.

In a building stabilised by shear walls or a core, the question is whether the existing elements have the capacity and whether their foundations can take the uplift. In a frame stabilised by moment connections, the question is whether the connections were ever designed for that and whether they can be verified.

Lightweight construction for the new storeys helps on vertical load and helps less on lateral stability, because wind load depends on exposed area rather than mass. It can also make things worse, by reducing the dead load that resists overturning.

This is the point where schemes lose a floor. One additional storey often works on an existing stability system. Two frequently does not without new shear elements, and new shear elements need a continuous load path to foundation level, which means running them through occupied flats.

Lift, stair and service cores

The core is the practical constraint that decides whether the scheme is buildable.

The existing stair has to serve the new storeys and has to comply with current requirements for means of escape at the new building height. Extending a stair upward through the roof is usually straightforward in structural terms and difficult in logistical ones, because it means the only stair in the building is a construction zone.

Lift extension is harder. Shaft structure, overrun height, machine room arrangements and current lift standards all have to be satisfied, and an existing shaft may be too narrow for a lift meeting present accessibility requirements.

Services need capacity checks on incoming electrical supply, water pressure at the new top floor, drainage stack capacity and soil vent termination, and heating plant if the building has a communal system. Water pressure is the quiet one, and a booster set has to go somewhere.

Temporary works above an occupied building

This is the section most reports skip and most residents care about.

Craneage and its oversail, scaffold loading onto an existing roof or independent scaffold from ground level, weather protection while the roof is open, propping, and the sequence that keeps the building watertight at the end of every working day. Each needs a position in the report even if the detail comes later.

Fire safety during the works is separate and critical. An existing block relying on a stay put strategy may not be able to maintain it while compartmentation at roof level is opened up, and the responsible person's fire risk assessment has to be revised for the construction phase.

Where the finished building is 18 metres or more in height, or has at least seven storeys, and contains at least two dwellings, it is a relevant building for Planning Gateway One. That brings the Health and Safety Executive in as a statutory consultee at the planning stage and requires a fire statement with the application, as covered in what a fire statement must cover at Gateway One. Adding two storeys is exactly the kind of change that pushes a building over that threshold.

Evidence at prior approval stage

Class AA applies to purpose-built detached blocks of flats of at least three storeys, constructed within a defined window of years, and caps the finished building height. The prior approval matters cover transport and highways, contamination, flooding, external appearance, adequate natural light in all habitable rooms, the impact on the amenity of the existing building and neighbouring premises, and air traffic and defence assets, with a construction management report required alongside the application.

Structural adequacy is not generally one of the prior approval matters. That surprises people, and it should not reassure them, because the Building Regulations question still has to be answered and answering it late is what kills schemes.

So produce the structural appraisal before applying, not after. Use it to fix the number of storeys, the construction system and the core strategy, and then submit a scheme you can actually build. A structural survey is the document that carries this work, and the limits of the class should be checked against the current text of the Order rather than any summary.

Where the scheme has to lose a floor

Be willing to reach that conclusion early. The usual triggers are foundation capacity that cannot be improved without underpinning, a stability system with no route to foundation level, a lift shaft that cannot be extended, or a height increase that pushes the building past a fire or planning threshold the scheme cannot afford.

One storey delivered is worth more than two storeys designed and abandoned. And an honest appraisal that says so in month one costs a fraction of the one that says so in month nine.

Enter your postcode on PlanScope to see which reports an upward extension would need alongside the structural work.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. A fire statement requires a professional with the necessary fire engineering competence, and a structural appraisal must be carried out by a suitably qualified engineer.

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