Extension electrics

Hello,
I'm getting electrics done for an extension - not built yet but getting there. Rather than changing the switch board at the front hall to take the extra wiring, can one bring a cable from the main board and put a second board in the extension and run everything off that. Or is there an official way of doing it by the book. Also, can I run a cable from the main board on the wall outside the perimeter of the house to the extension - so that we're not pulling the house asunder to get the cable to the extension.
Not doing it myself, just want to know what's allowed.
Thanks

Short answer to both questions is 'Yes'.

Obviously, rules and regs. apply so that the installation meets all safety requirements etc..

Great, thanks for that - saves the headache of taking the house apart