What happened
An extension's wall insulation was specified at 100mm rigid PIR. Site delivery records showed only 75mm boards arrived. By the time anyone checked, the plasterboard was on and the walls were skimmed.
How it was discovered
Final inspection. The U-value calculation in the Building Regulations submission could not be reconciled with the installed thickness.
The consequences
Two walls opened up to verify, additional internal lining added to achieve the U-value, redecoration. Significant cost and four weeks of additional time.
What homeowners can learn
Photograph insulation before plasterboard. Always. It is the single highest-leverage record on any extension.