Unless you use a contractor who is a member of a self-certification organisation (eg NICEIC or NAPIT for electrics; ie CORGI for gas, FENSA for windows) you will need to make a Building Regulations application for rewiring (Part P will apply to a complete rewire), new heating system (Part L), replacement windows (Part L?) ...
You will need Building Regulations application for any new connections to drains or sewers (which might cover putting in an en-suite) as AFAIK plumbing isn't (yet) covered by a self-certification scheme.
Owain