My parents are now in their 70's, and the house they lived in had a steep slope to the lot, and a bunch of stairs to go from the basement/garage up to the living level. I lived 30min away, and my brother lived 30 in the opposite direction...so if we needed to do anything for them, it would take half the day. When my dad took a tumble in the front yard, we decided it was time for them to move, and we wanted them close to one of us in case they needed us.
My brother lives in a garden home, with no room for expansion. My dad always used to tell me how much he liked my neighborhood since it's nice and quiet, but only a few minutes from "civilization". After discussing several options, I decided to add on to my house and have them stay here. After several iterations, we settled on a separate structure similar to a pool house. The layout will be open similar to a studio apartment, and will connect to my house thru a covered rear patio.
Here's the design, note the bathroom spacious and ADA compliant. We had originally planned to have the bathroom reinforced as a storm shelter, but we've decided to put a stand-alone shelter in the garage.
Here they have excavated for footings (ignore the shitty fence that the 'gated' community next door installed and let deteriorate)
Since I'm only adding one more bedroom, I can tie into my existing septic take. If I had added 2, I would have had to install a separate system.