Building Regulations exist principally to ensure the health, safety, welfare and convenience of people in and around buildings, and the water and energy efficiency of buildings.
The regulations apply to most new buildings and many alterations of existing buildings in England and Wales, whether domestic, commercial or industrial.
It's important to understand building regulations as you are responsible for making sure that the work complies with them if you are carrying out building work personally. 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. Complying with Building Regulations is a separate matter from getting planning permission for your work.