The chances are very much that it *is* their fault but you may find in
the small print somewhere that they waive responsibility if *you* do
the measurements?

IANAL, advice is worth what was paid for it.

You'd have a substantial chance of getting such a clause struck off, as it
is of sufficient unreasonableness IMO, to have required being brought to the
OP's attention.

They have contracted to do the design work. The design work (provided it can
be proved that measurements were correct) is demonstrably unsound. Clear
breach of contract.

Christian.