That's not a simple error, that's pure negligence.
Work out your additional costs (ie your losses), ignoring any presumption that the builder should have spotted the mistake, as that is not your concern, and then inform the Architect of your loss. Give him the opportunity to pay you by a set date (10 days), or else you will issue a claim in the county court, and also complain to his professional body if he is a member of anything.