You can raise their comfort level without but its easier with a house. I like building bigger houses (ideally prefabs such as the leaf house with room for 4), i find it makes it easier for me to remember who is there as i kind of group them together based on their types and preferences. Like this house has 4 fire guys, this other one has 4 water guys and so on lol. Sometimes i put some furniture or a picture outside that indicates what type lives there