Obtain all the building codes in your area, regarding the height restrictions and building envelope of your property. These range from district to district, so get a clear description from your local architectural review board.
Use the original blue prints of your current tri-level structure to begin adding additional spaces to expand the floor plan. If this blueprint is not available to you, you will need to measure out your current floor plan. Use a tape measure, graph paper and partner to diagram the home to scale on graph paper. Measure the outside first. Once you have the outside parameters outlined, create the interior diagram.
Add a sunroom to the dining room or off the living room. Choose an area that you can legally expand the floor plan, and fits within the parameters of your current design. Contact a local architect to help you design an addition that supports the look and aesthetic of your current home.
Add an additional car stall to your garage. This will add substantial square footage to your current floor plan. This addition will be relatively straightforward and easier to complete than a sunroom or a space indoors that requires insulation, drywall, paint and extra aesthetic features.
Submit your new floor plan to the governing authorities within your county for approval before undertaking construction. They will determine if the project is engineered properly, and fits within the set parameters of the building codes.