From PsyHelp

This is your default room, along with your Protected and Storage spaces.

All the stuff your pets invent, gather, find during adventuring, and craft will end up here.

To expand beyond your common room, you can create other rooms to help organize your stuff, and you can use certain blueprints to get your pets started on building new house extensions.

Many owners have a room for food pets can eat, a room to hold materials for crafting and inventing, a locked room to hold food for cooking (a "pantry"), and a locked room for profile items.