Typically, you’ll have a top-down diagram of the building, in your 3D software you can trace the walls with a spline and then extrude that up to the height of the wall. I try to make the walls of each room as a separate mesh.
And yes, you want to make both sides of the wall, so there should be some empty space between one side of a wall and the opposite side.
You should look up some house modeling tutorials, the concepts apply to any 3D software.