Material Safety Data Sheet (MSDS) define propane as extremely flammable liquefied petroleum gas.
Per article 500-3(a)(4) of the NEC propane is classified as Class 1, Group D.
Table 7902.3-A of 2001 California Fire Code (based on Uniform Fire Code) states that the minimum distance from outdoor storage to property line or property that can be buuilt upon is 50'.
Please contact Authority Having Jurisdiction for any code interpretation.