
Building regs prevent you from fitting a septic tank commensurate with the size of your (in our case, tiny) house. The minimum size you can get is frankly massive, about two thirds of the size of the Shack's actual bedroom.
This one - an 'Acorn' - is kind of lozenge shaped, saving our protected tree roots from excessive digging to bury it.
Try telling Adam - who has done the lion's share of the digging - that what he's done is not 'excessive'!