The owner carries the legal duty
Under the Building Act, the person commissioning the work — usually the homeowner — is legally responsible for ensuring it complies with the Building Regulations, regardless of who actually does the building.
Builders and designers carry contractual duty
Your contract with your builder and your appointment with any designer should pass back the obligation to deliver compliant work. The legal long-stop, though, remains with you.
Why this surprises people
Most homeowners assume the builder is on the hook because they did the work. They are — contractually — but the council writes to the owner, not the builder, when something is wrong.
Frequently asked
The current owner inherits the regulatory exposure. This is why Completion Certificates and Regularisations matter at sale.