Building Regulations are mainly concerned with how a structure is put together and how it complies with the various standards for habitable accommodation. The Building Regulations ensure health and safety, energy conservation and disabled access in buildings among many other things.
Generally you require building regulation approval for alterations of a structural nature both internally and externally. This would include any extensions to a property including attic conversions, the use of steelwork beams in structural walls, enlargement of existing window openings and below ground drainage.
Some exemptions exist for example porches and conservatories provided the glazing complies with the relevant standards, and certain detached non-habitable buildings.
Your application for building control approval pays for the Building Inspectors to check your plans for compliance with the various regulations and for the Building Inspector to visit your property at various stages of the work to ensure that your builder is adhering to the regulations.
If you are in any doubt about wether you will need Building Control approval you should contact Abbey Design, OnSite or your Local Council Department for advice before undertaking any works to your property.