Verify there is room to build on your lot. Local zoning laws typically specify what percentage of the lot can be built on. If there is no additional room to add on at ground level, consider building up.
Ask the local building department if there is a height limitation. Some building codes restrict height, which would make it impossible to add a level.
If your waste system is a septic tank, instead of a sewer system, check the size of the tank to see if it is large enough to accommodate the type of room you want to add. Some building codes base the size of the septic tank on the number of bedrooms, not bathrooms. So, you might have to add a larger septic tank in order to add another bedroom, but a den or family room might not be a problem. Some building codes define a bedroom as a room with a closet.
Verify your setbacks with the local zoning department. Before adding a room to your house, make sure it will fit on your lot. Typically, you must stay a specific number of feet from the property line. If yours is a corner lot, the set backs might be greater. And if there are utility easements on the property, there will be areas where you cannot build.
Measure the distance from a swimming pool. If you already have a pool, there may be restrictions on how close you can build to it. In some areas the pool's depth closest to the house is how many feet you must be from the building. For example, if the pool is six feet deep, you must stay six feet from the house.
Map out the location for your intended room. Using graph paper, map out your lot, keeping it to scale. Draw the existing house, to scale, on the paper. Draw how you intend to build the room. Verify that the prospective room does not violate any setbacks or height restrictions.
Consult a heating and air conditioning specialist to see if your current heating and cooling system adequately will cover the addition. If not, you might need to install a new unit or add a supplemental unit for the room addition.
Contact an electrical contractor to see if you would need to add electrical panels to the house for the new room.
Take the floor plan you've drawn to an engineer or architect to write up professional plans for the room addition. Or if you hire a general contractor for the project, that person might have someone who can complete this task for you.
Get the building plans approved by the local building department. When you do this, find out what the licensing requirements are for those who will build the addition. This information will determine who you can hire as well as what you could to do on the project.
Hire a general contractor to oversee the job, hire subcontractors to oversee the individual steps or hire laborers to complete each task. This would include someone to lay the footings and foundation; framing; electrical; plumbing; insulation; drywall; windows and doors; roofing; and painting. During each building phase, you must have the project checked and approved by the local building department before moving to the next step.