It is not a matter of "the builder" requiring specialist expert knowledge of the Building Regulations. The legal test is should an average competent tradesman know that building an extension to a residential property should comply with the Building Regulations? I would suggest that that puts the duty of care squarely on the builder's shoulders, especially if the customer is a member of the public with no expectation of knowledge of the Building Regulations.
There might be certain factors which make it exempt from the Building Regulations such as it being a porch or a conservatory but I would expect an average tradesman has a duty of care to flag that up and get the customer to get it confirmed.
My recollection of breaches of the Building Regulations is within 6 months of the work legal action is taken against the builder, after 6 months action is taken against the property owner. Although I think formal action is extremely rare unless the work is deemed to be dangerous. Building Control seem to rely on refusing to sign off work that does not comply as their only sanction and leave the homeowner to sort out the mess.
There might be certain factors which make it exempt from the Building Regulations such as it being a porch or a conservatory but I would expect an average tradesman has a duty of care to flag that up and get the customer to get it confirmed.
My recollection of breaches of the Building Regulations is within 6 months of the work legal action is taken against the builder, after 6 months action is taken against the property owner. Although I think formal action is extremely rare unless the work is deemed to be dangerous. Building Control seem to rely on refusing to sign off work that does not comply as their only sanction and leave the homeowner to sort out the mess.