Research · Paper 12 · August 2026
The same structural finding costs different amounts depending on when it arrives. Discovered before a layout is fixed it is a design input. Discovered after commitment it is a variation, and after installation it is a dispute. Nothing about the finding changes; only its position in the sequence.
The finding is identical at every stage. Its cost is not.
Layout is the cheapest variable and the first one to become fixed.
Roughly a third of roofs in our dataset needed intervention. Most were resolved by design change, not structural works.
01 · The sequence
A structural constraint on a roof exists from the moment the building does. When it is discovered determines what can be done about it.
The constraint becomes an input. Modules move out of the affected zone, tilt or ballast arrangement changes, or the array is sized differently. Across the 575-roof dataset roughly a third of roofs required some form of engineering intervention, and most of those were resolved by adjusting layout, tilt or ballast rather than by structural works. At this stage that adjustment is free.
The same adjustment is now rework. Drawings are revised, yield modelling is redone, and a commercial case built on a specific output has to be rebuilt on a smaller one.
It becomes a variation, with the cost falling wherever the contract puts it. This is where the argument about who should have asked begins.
The array must be removed to do anything. The cost is now the works plus the removal plus the reinstatement plus the lost generation, and the question of liability is live.
Figure 1
What the same problem costs at each stage
02 · Why it is discovered late
The installer is engaged to install, the designer to maximise yield, the funder to appraise a financial case, and the building owner has no reason to assume the roof is in question. It is nobody’s deliverable by default, which is precisely how it goes unasked.
Where a certification scheme applies, a check is prescribed at a defined point. Above that scope nothing prescribes when the question is asked, so it is asked when somebody happens to raise it — which is often at the point a lender or insurer reads the pack.
A scheme priced without an appraisal is cheaper on paper than one priced with it. The difference is not a saving; it is an unpriced risk that surfaces later, and later is where it is most expensive.
03 · What to do about it
A structural view at concept stage costs a fraction of one at variation stage and changes what gets drawn. It does not need to be a full appraisal to be useful at this point.
Conditional outcomes are the majority of interventions. They are solved with layout, tilt and ballast, and all three are cheap to change and expensive to revisit.
Where a lender, insurer or purchaser will read the documentation, an appraisal already in the pack prevents the late question entirely. That is the cheapest version of this whole problem.
How Solar Surveys approaches this
Every stage of a project makes a structural surprise more expensive to absorb. Solar Surveys is arranged around removing that surprise at the cheapest possible moment, which is before anybody has committed to a layout.
Where a portfolio needs narrowing before anyone travels, a desktop assessment sorts the obvious from the uncertain and concentrates site time on the roofs where it changes a decision.
The structural answer is most valuable while the design can still respond to it. Once modules, ballast and inverters are arranged around an assumption, changing that assumption costs a great deal more.
We aim to return the report within 48 hours of the site visit, because a programme that stalls waiting for structural input tends to proceed on the assumption instead, and that is precisely how the cost gets deferred to a worse stage.
The structural answer costs the same whenever you ask for it. Everything built on top of the wrong assumption is what gets expensive.
Solar Surveys Ltd · structural surveys and roof condition assessments for commercial solar PV across the UK · £5m professional indemnity cover in force.
04 · Basis and limitations
05 · Citation
Suggested citation
Raihan, S. (2026) The Cost of Finding Out Late: When a Structural Finding Arrives. Solar Surveys Research, Paper 05. Solar Surveys Ltd. Available at: https://solarsurveys.co.uk/research/the-cost-of-finding-out-late
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