And the primary responsibility for achieving compliance with the regulations rests with the person carrying out the building work.
The person carrying out the work must comply but the it is the responsibility of the person employing them to ensure that they do comply.
Been there and personal experience is that the property owner is the person the LABC will take action against for non compliance. It is then up to the property owner to take it further against the person whose hands did the work.
( no action has been taken against me as nothing contravened in the house we self built but I needed to be 100% certain so I spoke to the LABC officers before starting the renovation project which may involve trades people being employed )