Here are some ground rules:

• The thing has to be unnoticeable: keep the dumb switches on the wall and no more tape over the one that powers a wireless bulb. In case the network goes down - yes, even the home network and not just the Internet connection - it still has to work properly. In case the microwave oven is running, it still has to be reliable.
• It has to be easy to maintain: use dumb lights, stock some cheap replacements and be able to go to the store nearby to pick up a new one when needed.
• It has to be wired: this is a faster and more reliable way of control with much less interference to worry about.
  Wireless is nice and some ambient lights are already Zigbee controlled. They are permanently installed, powered all the time and this even improves the wireless mesh network. WiFi is used for seasonal lights, such as holiday decorations or pool lighting.
• It has to be made with hardware that I have at hand: I have a huge stockpile of various electronic boards all waiting for to be built into something. I'm sort of a hoarder in this regard and space is running out slowly, so it's about time to put the collected parts to use.
  Of course there are some exceptions, such as power supplies, proper relays and enclosures. Yes, this means DIN-rail mountable enclosure for the controller and DIN-rail mountable relays for switching high voltages.

And the goals:

• Have a WAF of 100%. Or at least a lot of nines after the 99 and the point. Not just the wife, but the kids, parents, guests, non-technical people and even the future owners of the house (if it ever gets sold) have to be able to switch the lights just as before. We should just get an extra option for remote control, status updates and automation.
• If successful, use it as a template for other similar projects. Keep the microcontroller board, get a new I/O board and be able to use it in some other location. Or change the microcontroller but keep everything else, especially the software components (with some configuration change). Keep every piece of software modular, easy to maintain and expand.