First, go to the city code enforcement officer. The officer can tell you where the utility easements are and what laws affect them. These easements, i.e., alleys if they run between the back of adjoining properties, are usually 15 ft. wide, 7.5 ft. is granted from each property owner. Technically the property owner owns this property but has granted its use to the city for utilities, i.e., water, gas, power poles, sewer line, etc.

The code usually says the the property owner can use this property as long as he does not deny access to the city for utility maintenance. Building on top of a sewer line would be denying the city access. Property owners can build a fence across their portion of the easement but the city has the right to remove the fence for utility maintenance.