Your neighborhood probably is subject to legal constraints. Before you begin the design phase of your addition project, be sure to check with city and county offices to see what kinds of restrictions affect your property. Now is a good time to reread the fine print in your deed and abstract for the property.
The legally buildable portion of your lot doesn't stretch to the lot lines; instead, it occupies a "bubble" of space that sits several feet within the lines on all sides. Thus, your buildable area is subject to a front setback, a rear setback, and two side setbacks. The purpose of these inner boundaries is twofold: to ensure adequate open space between neighboring structures for fire-safety reasons, and to maintain a uniform streetscape and therefore establish and maintain the street's basic sense of place. On some lots, the bubble is further reduced by easements. Easements preserve strips of open space on private property to provide occasional access for nonprivate purposes. An example is an easement along one side of a lot that abuts a utility trunk line at the rear; the easement ensures legal access from the street for servicing or repairing the trunk line. Because easements often fall inside setback lines, it's important to know if any are in force on your property before you break ground. If your addition should encroach on an easement, you would have to demolish the illegal portion of the structure, which may mean reconfiguring the entire project.
If you're planning to build up, or if part of your addition will tower over your house or your neighbors' homes, be sure to check with local authorities regarding rules governing the height of new structures in your area. Such rules serve several purposes: to ensure that ladders on fire trucks can reach the upper floors of residences in an emergency, to maintain a uniform sense of scale or streetscape, to protect view rights of nearby residents, and to preserve access to direct sun for adjoining property owners.
In many communities, the buildable bubble on each lot is restricted by a buildable-area ratio (BAR) ordinance. BARs set a maximum allowable ratio for the total amount of built space versus unbuilt (open) space within the bubble. This ratio varies from community to community, typically ranging from 5:10 to 7:10. In other words, if for some reason you wanted to build an addition that enclosed all but a few square feet of your backyard, it's unlikely that local ordinances would allow it. Again, the reasons for such a restriction have to do partly with public safety and partly with maintaining the character and livability of the neighborhood.
Every region of the country has at least a few communities where unusual rules or limits have been adopted because of local conditions or circumstances. One example is an ordinance that protects the character of historical neighborhoods. Another is a set of covenants (special agreements or standards) that establishes and maintains a particular architectural theme throughout a neighborhood or town. There are ordinances that prohibit on-street parking and limit the number of vehicles that can park within sight of the street—a rule that might cause concern if your addition will displace your garage or a big chunk of your driveway. Communities in areas prone to forest fires have adopted codes that specify the type of roofing material you can use, and structures in areas that are subject to frequent tremors must satisfy special codes that make buildings extra resistant to wracking, shifting, and swaying.