Use heating oil tank as diesel tank?

Diesel does not burn. It also does not explode. You can put out a burning match with it.

I know, that's why I drive a gasoline car! But seriously, you can't store 6 liters of diesel as fuel, who is supposed to fill that, how do you refuel? Environmental impact? etc. And again: it is a fire load.

And btw: if diesel really wouldn't burn at all, that would be quite fatal for many.

Diesel ignites under pressure, not by sparks. Therefore, the liquid is not hazardous in terms of fire risk. Since heating oil was classified as non-flammable, the requirement for fire extinguishers in houses with oil heating was also eliminated in 2018. The necessary pressure required to cause diesel to explode can hardly be exerted on the tank. Despite technical safety, I would be very surprised if anyone would approve it. Have the tank removed and do something useful with the gained space.