Not Greek, but here's an economics Stackexchange question which handles this:

https://skeptics.stackexchange.com/questions/27662/do-greeks...

tl;dr of the accepted answer: It's not for tax evasion, it's to make later expansion cheaper. They don't count as "Unfinished buildings". If the house has power (empty or not), or someone lives inside (with or without electrical power), the owner must pay the full 100% of the normal property taxes.