Do I need to fill out a tax return if I buy to let without a mortgage?

I can afford only to rent in the area where I work, so I plan to buy a house for cash elsewhere to let to my sisterQ I have a query regarding home-owning and was hoping you could help. I have rented for my whole life and have never taken out a mortgage or owned a home. I am required to stay in my rented house for my job. However, I am now in a position to buy a house in a much cheaper area for cash, which I could rent to my sister. As I won’t be taking out a mortgage, am I still required t