So, just so I’m understanding correctly, unless the ADU has a driveway for parking of its own, then rent rates might go down on the actual house bc, essentially, the property land is being shared by different parties?

Lots of people rent houses for parties as well, and you won’t be able to do anything about it.  

Blood gets thick and the worst part is 10 years later there are new voices in their ears, telling them all kind of crap and filling it with all kinds of ideas. 

Quote from @Jay Hinrichs: Quote from @James Hamling: Quote from @Jay Hinrichs: Quote from @James Hamling: Quote from @Jay Hinrichs: one  way i know to replace 350k a year with 750k is to build new construction and sell it.we pretty much double our money COC with new builds within 12 months.So you pay cash for the lot  get a construction loan and the hard part is going to be a GC you know and trust.. sell the house double your cash on cash or better.. pay your tax and keep going.Or invest with a developer and do value add by creating building lots those are easy to 2X or better more risk though.

Happy you joined the party.. how soon do you plan to start crunching numbers on a potential deal?

As far as benefit to the seller goes, @Matthew Paul I think you are probably right on the money and regardless of whether they would have preferred to stay in this house rather than move, it is likely providing them some excellent cash flow; and I'd be hard-pressed to come up with a deal that serves both parties better than the current situation. 

However, you can make changes as long as all parties agree.If the tenant agrees to removing certain clauses, or starting with a new lease, then you can move forward.

This includes credit, criminal, and rental history checks for all parties.

Or do you have to use a 3rd party site to keep them updated and then spend more? 

Whether that is a sale to you or a 3rd party I don't know but the threat alone may be enough for her to sell it to you.