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.