There are many laws in place regulating where and what you can build. Even something as simple as making a small addition to your home, will require a building permit. Building permits can be obtained at your local building department. At the building department you can find out what you need to file, and what will be expected of you. If you are uncertain about whether you need to obtain a permit or not, going to the building department will be helpful for finding out more. If you are hiring a contractor, however, it is easier to have him request the permit, since he most likely has done so many times and is familiar with the process.
When you buy a building that has had work done without a permit, you could end up the one responsible for fixing problems, even though you weren’t the one having it built. If there are recent modifications to a building, it doesn’t hurt to request to see the permit obtained for the project.