Jeroen,

In a household situation, having isolated 485 comms is overkill. I done a bit of lighting control over the years and have installed systems with many nodes spread around a building using RS485 and not required isolation. Even much of my instrumentation systems that have 1km of cable and a number of nodes operate quite happily without isolation.

If you want to use transformers, you need to choose an encoding scheme that won't introduce a DC offset, otherwise the transformer will saturate rendering any communicaion void. So you would have to use manchester encoding or other means to achieve this. This rules out using the uart then! It might just be cheaper to go ethernet vs isolated 485 - is would be getting close in cost methinks with ethernet getting cheaper in the longer term - the transformer/RJ45 being a mass market item. Many processors have the mac/phy built in (PIC,Luminary, and there's a 8051 based chip that escapes me at the moment).

Then you've also got the option of power over ethernet.