Generally, NO permanent structures (including foundations, footings and overhangs) are allowed within a permanent easement; fences (without footings) are allowed; planting trees and shrubs are discouraged from the standpoint that if utility work has to be done within the easement, the plantings may be removed and are not required to be replaced.  However, there may be special considerations for individual easements, and therefore, the easement document(s) should be reviewed for details.