A wall is a "party wall" if it stands astride the boundary of land belonging to two (or more) different owners.
Some kinds of work carried out to a property may not be controlled by the Building Regulations, but may be work which is covered by the Party Wall etc. Act 1996. This is a separate piece of legislation with different requirements to the Building Regulations.
A booklet has been produced by DCLG to explain in simple terms how the Party Wall etc Act 1996 ("the Act") may affect someone who either wishes to carry out work covered by the Act (the "Building Owner"), or receives notification under the Act of proposed adjacent work (the "Adjoining Owner").