Building regulations - the approved documents
The full set of English and Welsh Building Regulations define how each part of a building needs to be constructed or altered - there is no need to upgrade an existing building to the current standards.
However, some aspects may come into play if the use of the building is changed i.e. from a family house to a house of multi-occupancy - generally such changes will need other local authority approval anyway.
The Scottish government have Technical Handbooks which provide guidance on achieving the standards set out in the Building (Scotland) Regulations 2004 - these are available in two volumes, Domestic buildings and Non-domestic buildings.
Building Regulations are completely different from Planning Permission, you need Building Regulations approval even where Planning Permission is not required.
The Local Authority will have a Building Control Office; this office will need to confirm that any new work is done in accordance with the Building Regulations, you will need to pay a fee for their approval and site inspections, but if you do the work without authority/approval, it is a criminal offence and you may have problems selling the property without the appropriate certificate.
The National House Building Council (NHBC) can be employed instead of the Local Authority to inspect/approve the work and provide a 10 year guarantee on the work they approve; however, this isn't a realistic option for the diy'er.
The most recent versions of the Approved Documents of the Building Regulations are listed below and are available from the Planning Portal for either purchase or download. It will be noted that some of the documents have not been updated for sometime while others have more recent editions - it pays to keep an eye open for updates but any applicable change will be highlighted by the local Building Control Office.
Approved Document for Regulation 7