If you are paying your monthly subscription, you should be able to keep your house no matter how long between log-in's. However, if you do not keep your game subscription current there should be a grace period (say six months) before the house is forfeited.

IMHO were you to lose the house, the permit and building cost should be refunded and all items contained should be put in a special storage that would allow you to sell them when you come back (if you are unable to find a new plot to build).