As others noted, you definitely MUST follow electrical code guidelines for something like this, so you have to start there.
However, for something like this, it seems the best way to go is a distributed system, with communications between multiple smart nodes. The code may specify the sort of comms busses that can be used in houses, but I am thinking a CAN buss might be good, as used in autos. RS232 won't cut it for long distances, but RS485 may, and CAN buss is really popular now.
As I understand it, in the old days they used to have the audio/etc links to the seats in passenger airlines all individually wired back to a central hub, requiring miles and miles of copper, but then they went to using a comms buss arrangement with local processors in each seat, so they could get by with stringing only a simple daisy-chained connection from seat to seat.
Sounds like a good way to automate a house.