May I install an underground tank under a car parking space?

This is generally possible, subject to fulfilment of two conditions. Firstly, the tank must be able to support the weight of the vehicle, which will require costly modifications. Secondly, the tank cavity has to be constructed so that it is gas-tight and a discharge outlet pipe will need to be installed at a sufficient distance from the car parking space. Again, this feature will cost substantially more than a standard tank. This option only makes sense if there is genuinely no other place to locate the tank.