Do you have to advise about the tenant prior to mortgage? Slightly underhand I guess, and I wouldn’t like to rely on family paying if everything went wrong - but if I knew I could cover all mortgage costs regardless and would be happy to do so, then I might consider a buy to let mortgage. Years ago I rented a place off my parents that they had a buy to let mortgage on, in their retirement, but I wasn’t aware of any restriction and I’m sure they weren’t that clued up either.