What happened

Rear extension windows were ordered to a thermal specification compatible with the U-value calculation. A communication error meant a lower-spec unit was supplied and fitted.

How it was discovered

The FENSA certificate could not be issued because the installed product did not match the Building Regulations submission.

The consequences

Units replaced. Four weeks lead time. Disposal of the wrong glazing. A material cost the supplier and installer eventually shared, with the homeowner caught in the middle.

What homeowners can learn

Check the product datasheet on delivery, not at installation. Returns from a builder's yard are easier than returns from a finished wall.