With all building work, the owner of the property (or land) in question is ultimately responsible for complying with the relevant planning rules and building regulations.
If you want to carry out building work, you may require both Planning Permission and Building Regulations approval. Planning and Building Control are separate; if you get one you are not guaranteed the other.
Planning has more to do with the appearance of a building or an extension to a building and how it relates its surroundings.
Building Control aims to see that the building satisfies its functional requirements and provides reasonable health, safety and comfort for the building users.