If your accessory structures are within 30 feet of your home, they should be mitigated to the same extent as your home. If they catch fire, they can set your home on fire as well.
Ideally, propane tanks should be located at least 30 feet from any structures, preferably at the same elevation as your home. Vegetation should be cleared underneath and 5-10 feet around your tank. There should be no combustible fencing around your tank as well.