The railing at the staircase has to be removed to get certain pieces of furniture down, and I don't want to do that every time there is a turnover.

@Nancy BachetyWe have one we use for our vacation cabins who covers the outdoor pool and other potential liabilities.

Nice furniture, maintained, clean, no signs or smells of cats, just pride of rentership.

The sheltering need not be clandestine, and harboring covers aliens arrested outdoors, as well as in a building.

But you can certainly mandate the use to be outdoors, and I would require a separate deposit for potential odor removal.

OR I could build it out to rent the main house and the finished basement (sans kitchen/furniture etc) to one tenant and make my life easier. 

I usually leave a welcome card with things like a 2-pack of HVAC filters, the assorted pack of furniture felt pads so they don't scratch my floors with their furniture, stainless steel spray or special cooktop cleaner if needed so they know I want them to use the recommended product.

., furniture in storage or sold, and then either sell the house or rent it long term.

I think you'd be surprised how much just some basic soft goods like furniture and sofas help with that kind of Noise though, so if both unit are currently vacant I am willing to bet it sounds much worse than when people will actually live there and have curtains and carpets etc.

He may even be able to eek out a little more rent on the 2/1 units, since they are newly renovated, with popular style/design and depending on the outdoor space and onsite parking available.