What advice would you give to someone rewiring a home from scratch?

Hey all, based in the UK.

A family member has just bought a home and is planning to rewire it. They've seen my HA setup and are very keen to do similar - but my setup is a mesh of things that have mostly been "tacked on" after the fact (relay switches, smart plugs etc).

They are in the rare position to be able to do things in walls with relative ease, and I'd like to be able to help them plan that (and avoid kicking themselves later like I have).

So I thought I'd draw on the wealth of experience in this sub. If you had the opportunity to redo some things from scratch, what would you have planned to do?