When working on a property which is semi-detached or the work is close to a boundary or an adjoining property, you will probably need a party wall agreement between you and your neighbour. In most cases, this is a simple process completed by an exchange of letters agreeing to the work.
Complex projects often require the services of a party wall surveyor to oversee this process. They are normally a member of the Royal Institute of Chartered Surveyors who have their own code of conduct and are required to have the correct insurance, training, and expertise to undertake this role.
More information on the Party Wall Act can be found here.