The coziness

effect increases the amount of Craftsman's insight and Battle focus points you receive at server's reset time.

To get this bonus you need a house and log out being inside it to get its bonus:
 * 10 % bonus - Basic shelter
 * 20 % bonus - Timbered house
 * 30 % bonus - Half timbered twin inn

You can safely remove roof and walls of any house without loosing its bonus.