In my state it is 12 feet in any direction needs a permit, but every 'structure' no matter what still has some rules as you mentioned about property lines.

Anyway codes don't seem to care about what is under pt runners for accessory buildings in our area even though it freezes good, but they do care about adding earth anchors and straps to keep the thing from blowing away. To me the gravel under the runners is more about keeping the floor from rotting than frost heave issues. An uneven floor in a shed is easily fixed, but not a rotten floor.