The requirements are strict. If you get caught storing fuel underground, the penalties are tough, the fines high. But there are alternatives. They are inexpensive and durable. But you'll need a lot of them if your fuel supply needs are great.
Next option is auxiliary tanks for vehicles and pickups.
Many truck supply companies manufacture tanks that fit in pickup beds. Consider 55-gallon steel drums. These must be stored in a well ventilated area away from heat. Large surface storage tanks with capacities of 250 gallons plus are the best alternative. Omega tanks also has large water storage tanks.