It really depends on the layout of the property - how to get to the ADU without tenant problems matters the most. 

Unfortunately no way to use a common area with the current layout .

Putting a "Beautiful Floor" in a rental garage is asking for trouble.If you use delicate type things (easily stainable or easily breakable) in a rental don't expect them to hold up.Cars park there--and cars make messes--leaky oil gaskets, radiator hose broken (rust deposits), rubber tire marks.Maybe you could call the product manufacturer and ask if there is a way to clean off normal car stains.Good Luck!

Kitchens can be trickier, depending on the layout.  

New ContactsBeing a real estate investor and Realtor can be a delicate balance.

Amazing how high hats and removing hallway headers can change the layout of a house.  

They will scrutinize your material, layout, and contractors. 2nd, if any layout deviates from the original plan, you have to find a structural engineer to prove to them the change won’t compromise the structural integrity of the building. 3rd, your finished materials especially flooring must meet their sound proof standards.

You just need to basically lay out the agreement for the loan in the promissory note.

I understand there is a family dynamic here, but you could offer it delicately like, "I'm not expecting you to accept this, but this is the most I am comfortable paying in order to make the numbers work".

I did a similar thing by buying a SFH with a nice layout to rent out the two spare rooms.