@Account Closed said about slowly upgrading your tenants and starting with the worst first.
You are also right about the hoarders. Hoarders don't change without a lot of counseling and support. I would know... I'm a recovering hoarder. It has taken me 5 years to sort through and part with my bus loads of junk. I am finally at a healthy place. It is a slow, tedious, and emotional process. There is no quick fix. You could rent a dumpster, which might encourage them to part with some things, but it will not solve the problem and won't help you at all with how quickly you need it remedied. I do appreciate the thoughtfulness behind the suggestion of course. As someone who has been in that state of mind though, it isn't a good solution because hoarders don't see their items as trash or junk. They won't willingly part with enough to make a difference unless they aren't given the choice.
It is sad to evict tenants who have been faithful renters for so long, but infestations are bad for everyone, including them. Be prepared for an uphill battle because moving at their age is difficult. It's a lot of stress and work, even if they hire help. This is a tough situation but I wish you the best of luck and think you are on the right track. Keep us posted! I'd love to hear how it all pans out. And congrats on your new multifamily building!
