The Building Regulations apply to building work in England and Wales and set standards for the design and construction of buildings to ensure the safety and health for people in or about those buildings. They also include requirements to ensure that fuel and power is conserved and facilities are provided for people, including those with disabilities, to access and move around and inside buildings.
Compliance with the standards of construction prescribed by the Building Regulations is a legally enforceable requirement.
It is therefore important to understand the building regulations as any person who carries out building work has a duty in law to comply.
If you are employing a builder, the responsibility will usually be theirs - but you should confirm this at the beginning. Also bear in mind that if you are the owner of the building, it is ultimately you who may be served with an enforcement notice if the work does not comply with the regulations.
The Local Authority has the duty under the Building Act 1984 to enforce the regulations, and in the proper performance of that duty it needs to verify new building work by inspection.
Remember that complying with Building Regulations is a separate matter from getting planning permission for your work.