Evidencing load capacity for added storeys

Before anyone draws an upward extension, somebody has to answer whether the building below can carry it. On an existing frame that question has three parts: what the original structure was designed for, what it is actually made of now, and whether the foundations can take more load without unacceptable settlement. All three need evidence, and on a building from the 1960s or earlier the records usually do not exist.

The planning submission does not need full calculations. It needs enough structural appraisal to show the massing applied for is deliverable, because an authority that grants permission for two extra storeys on a building that can carry one has granted something nobody can build.

Why capacity is the first question on an upward extension

Adding storeys changes four things at once. Vertical load increases through every column, wall and footing below. Wind load increases, and with it overturning moment and the demand on whatever provides lateral stability. The consequence class of the building under the Building Regulations may rise with storey count, which changes the disproportionate collapse and tying requirements for the whole structure rather than just the new part. And crossing 18 metres or seven storeys brings the building into the higher-risk category with its own gateways.

Any of those can stop the scheme. The fourth is the one that most often turns a three-storey addition into a two-storey one.

Records, intrusive survey and what each gives you

Start with the paper. Original design drawings and calculations, if they survive, tell you the design loadings and the intended load paths, which is more useful than any survey. Building control records held by the authority sometimes hold them. For a building of any age, the local archive, the original architect's practice successor and the freeholder's own files are all worth asking.

Then open up. A survey programme on an existing frame usually includes:

• Trial pits to establish foundation depth, width, type and the bearing stratum
• Opening up at a sample of columns and beams to confirm member sizes and connections
• Cover meter and radar scanning, with local breakout, to establish reinforcement arrangement and cover
• Concrete cores for compressive strength, and carbonation or chloride testing where durability is in doubt
• Steel sampling or hardness testing to establish grade, because assumed grades on older frames are frequently wrong
• Masonry testing where loadbearing walls are carrying the load

The pattern is that each method answers one question at one place. A credible appraisal states how many locations were tested and why that is enough.

Load paths through an existing frame

Continuity is everything. A column that stops at second floor over a transfer beam, or a loadbearing wall that changes position where the plan changes, creates a point where new load cannot simply be added above.

Older frames also often rely on the floors acting as diaphragms and on infill masonry for stability in ways the original calculations never stated. Removing a wall during conversion, which the project may want to do for other reasons, can take away the stability the new storeys depend on.

What the appraisal has to do is trace load from the new roof to the ground at every vertical element, and say where it cannot be traced. A short structural report with a load path diagram and a list of problem locations is more use at planning stage than a long report with no diagram. There is a related piece on structural appraisal of a building proposed for retention.

Foundation capacity and underpinning options

Foundations constrain more schemes than frames do, because strengthening them is the hardest work to carry out in an occupied building.

Options, roughly in order of disruption. Accept the increased bearing pressure where the original design was conservative and the ground is competent. Widen pad or strip footings. Underpin by mass concrete or beam and base. Install mini-piles or needle piles through the existing structure. Or introduce a new load path that bypasses the existing foundations entirely, with new piles outside the footprint.

Differential settlement between strengthened and unstrengthened areas is the technical risk, and it is the one that produces cracking complaints from occupiers during construction. Where the ground itself is in question, a Geotechnical Assessment establishes the bearing capacity the appraisal is relying on, rather than leaving it as an assumption.

Lightweight construction and its limits

Light gauge steel framing, cross-laminated timber and panelised systems all reduce added load, sometimes enough to make a scheme work that would otherwise fail. They come with constraints.

Restrictions on combustible materials in the external walls of relevant buildings limit timber options at height, and the threshold heights have changed more than once, so check the current Building Regulations position rather than a precedent from a few years ago. Acoustic separation between the new storeys and the existing top floor needs designing rather than assuming. And a lighter roof changes the uplift case, which on an exposed site is a real design condition rather than a formality.

Lightweight also does nothing for the wind load problem. Reducing mass reduces the vertical demand and leaves the lateral demand where it was.

What the planning submission should contain

Proportionate is the word. For a planning application the authority needs to be satisfied the scheme is deliverable, not to approve the engineering.

1. A structural appraisal at feasibility level, stating what records exist, what survey work has been done and what assumptions remain.
2. A load path statement identifying any location where strengthening will be needed.
3. Confirmation that the foundation strategy is feasible, with the geotechnical basis stated.
4. Any external consequence of strengthening shown on the drawings, including thicker walls, new external columns or a widened core.
5. The fire strategy position, because a taller building changes the escape and access requirements before it changes anything else.

A Structural Survey is the document most authorities expect, and a Fire Statement becomes a national requirement once the building falls within the relevant category. Both need authors with the relevant competence: the fire statement in particular is a fire-engineering document, and a licensed or competent professional is legally required for it rather than merely advisable.

Where the economics fail before the engineering

Several schemes are structurally possible and financially pointless. The costs that accumulate:

Strengthening works in an occupied building, carried out out of hours with protection and temporary works. Loss of lettable area at ground floor where new columns or a widened core land. Lift extension, which on an older building often means a new shaft rather than an extended one. Stair widening or a second staircase. Decanting or loss of income during works. And the professional fees for an investigation programme that may conclude the answer is no.

That last cost is worth paying. An investigation that stops a scheme at feasibility is cheaper than a permission nobody can implement.

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, including structural and fire engineering work, qualified professional input may be required.