Remember that Intersect Faces thing. It will come in handy for a lot of things.
A couple of additional notes about your model. If you intend to extrude the walls, delete the faces for the floors first. If you don’t the bottom faces of the walls may not be made. Generally it is wise to wait to apply materials/textures until after you’ve made the structure 3D and ensure that all faces are correctly oriented. Get in the practice of keeping white front faces out and the blue back faces on the inside of the 3D shape. It’s OK to see those back faces on 2D faces because they generally sort themselves out when you go 3D but correct them as you go to prevent problems later. You can right click on reversed faces and choose Reverse Faces from the Context menu.