Spaces are areas that can be accessed from portal items that you can purchase through bundles in the shop in-game. Once you obtain a portal you can place it in your house and walk through it to start customising your space with furniture and unlock plots to have even more space!

You should be able to find it easily in your "Utilities" tab in decor mode:

Bundles or packs that contain an item of a space you already have should give you another portal for another space of the same type, so each time you purchase a new space you will have a brand new space to decorate! If for whatever reason you've obtained another space but it takes you to a space that you've already been decorating, let us know here and we will be able to help!