I just noticed that its is landlocked too.
So some type of shared right of way would be needed or the council allow a new domestic road opening on land you would also have to buy as a separate lot.
To get a basement the shed would need to be knocked, the slurry tanks dug out, deeper dig to add the pool 1.5 stories down, the back of the shed landscaped down into the hole....
That's an expensive site.