How do you see this situation as different from splitting rent with a housemate?

To me, the ethical questions lie not in the situation, but in how you handle it. Charge your tenant high enough rent to cover the full mortgage, maintenance, and renovations of the whole building? Sounds sketchy. Charge “market rate” just because that’s what the hedge-fund landlords demand? Sketchy. Find a rent level that accounts for your costs & maintenance of the rented space, an allowance for vacant time, and risk of bad tenants? That’s a great way to help people find housing, without being an actual charity, and make more sustainable use of a big space than living in it all by yourself.