Decide what kind of living space would really fulfill your needs and if you would need to do any renovating or updating of wiring, systems or other rooms in the house to work with the addition.
Get a survey done so you know exactly where your lot lines and house are situated, and how much space you have to work with. Also have the surveyor mark where any main utility lines, septic lines and tanks, and wells are located.
Check with zoning regulations in your area and any building codes to see what permits are needed. Your municipality or development may have minimum setbacks, a minimum amount of space between any building on the lot and the lot line. You may have to get a zoning exception as well.
Investigate if there are any additional rules and guidelines, like those in covenants, if you live in a subdivision, development or historic area. The guidelines or covenants may also require any plans for additions or alterations of your property to be approved by a homeowners association or architectural review board.
Find out what the selling prices are in your neighborhood and the size of the largest homes. You do not want the biggest and most expensive house in the neighborhood or you may not be able to recover the cost of your addition in the future sales price.
Consult an architect to draw up or aid you with finding the right plan and design for your house. Your addition should add, not detract, from your house and be in harmony with the neighborhood. You may also wish to consult a landscape artist to come up with the best design for your new home.
Interview licensed and bonded contractors, talking to at least three, and getting all bids in writing. If the bids are not close, make sure that you are comparing similar quality jobs, materials and results. If there is a bid that is much higher or much lower, throw it out.
Ask how long the job will take at the outset and be sure you can accommodate your family's needs during the construction period. For example, if you are doing an addition to the kitchen, you may be without one for several days to a few weeks. A new bathroom may require the water to be shut off for long periods, which would mean finding alternatives for using your shower, sink and toilet.