Oh, there's an old boiler down there too.
You need a granny flat not a basement
When we had a basement it was mostly full of water!!
The house in question was self-built job and the family concerned had not installed any perimeter drains on the hillside plot. So when it rained ,as it did in the winter of 2000 or therabouts , our cellar filled with water so rapidly that we called the fire brigade.
They were so busy in York and Selby that we were long way down the list of priorities, so I went to a garden centre and bought a pump for making a fountain (I think it was the only functioning pump available in Yorkshire at the time) .
After several hours of nocturnal pumping (ooh err, missus) the water level subsided to a level at which I informed the FB that they were no longer needed. They were more than happy to cross me off their to-do list.
I ended up hacking through 6 inches of concrete to create a sump and installed a pump controlled by a float switch, as ground water was a constant problem.
I have never trusted a basement that hasn't got a full bunding , drainage and ventilation set up since then.
The previous owners informed us that their sons used the basement as an indoor air rifle shooting range.