The options for communicating sensor data in a medium size building
Power line transmission is a good option for this. The chip-sets do seem to be able to handle "decent" data rates with a good degree of reliability. The 100kHz band looks probably the most attractive and is, as far as I know, dedicated to this sort of thing. This document from NXP (entitled AN10903, TDA5051A ASK power line modem) is a good reference for what you have to do. It contains a wealth of knowledge on how to couple to the AC lines and how to impedance match etc.. The chip-set is good for 1200baud and I think this would be good enough for what you are looking to achieve. Remember one thing, faster data rates = wider bandwidth = more received noise = less sensitive  receiver.

Here is the data sheet for the chip and below is a diagram in that document: -