If you are in the US, take a look at NEC Article 514 and look at references to Liquified petroluem gas. Propane is heavier than air.
I'm assuming you are talking about an outdoor location. If indoor, the area with propane storage would fall under Article 501.
There may be some additional NFPA requirements I'm not familiar with. You could try searching the NFPA website for "propane".