The potential for building a fence or retaining wall in the wrong area is huge and using the old fence as a reference can be highly inaccurate. The simplest way to know for sure where your property lines are is to have a Land Surveyor locate and mark your property lines for you.
A second option is to use the most current and updated Real Property Report for the property. This will show the home’s property boundaries and should also provide a clear measurement from the property boundaries to the dwelling. Simply measure off the distance from the dwelling to the property lines that are shown on the Real Property Report. This will give you the whereabouts of your property lines. If you are not comfortable with this process, then definitely have a Professional Land Surveyor mark out the property lines for you.