How much room do espaliered fruit tree roots need?
Tree roots will grow as far as they can in favorable soils. Tree roots will not grow into open air as air does not provide a favorable environment for root growth.
I would plant the peach tree no closer than 10 feet from the ledge. A ten-foot spacing should allow the tree to develop a good root system completely around the trunk. A tree with a good root system should be quite stable. A tree with poor root development on one side (for example, if planted close to the ledge) may not be very stable and could be uprooted (blown over) in a windstorm.