This is a great thread. Everyone has a piece of the puzzle. No two situations or solutions alike.
Re: basements and lofts
Age in place
Single level living
Worth thinking about
When you're young you don't consider worn out knees & hips, and falling off ladders.
Stairs, clutter and too much maintenance were the primary house hardships for my elder clients when I worked as a case manager.
In VT we are experiencing flooding more and more and so basements are becoming a liability. Plus many homes are sitting on ledge.
Recent mandates to "densify" and greater acceptance of ADUs has increased the ability to have a live-nearby helper when the day comes.
I'm a curmudgeonly loner and have a hard time imagining the day I would "need" a protective presence but am practical. So...
Plan A:
I'm living now in my future ADU. It will be an income-producer when I move into my larger home as a guest house, rental, AirBnB. Thus acclimating me gradually to having other people around. It's really hard to suddenly have a stranger in your space when you are 80. Better to work up to it.
Then, when the day comes, it can be a Home Share situation where a person pays a minimal rent and trade the rest as helper.
In the larger home I'm building the open concept (as several people have already mentioned here) because I know my needs will change over time and I want the space to be flexible. No interior fixed permanently - divider walls using screws so it can be altered. Beds do not get their own room, but are berths tucked into the walls and hidden.
Really high ceilings and lots of light make the smaller footprint tolerable. And they allow for a free standing loft area for storage if desirable. My "big" room will have a 21' high roof line.
Re: Building permits. In our town the building permit is for 2 years during which time you have to start something. I'm in year 2 and already know I'm not going to be done, so opted for the 2 year extension in advance. They can see I'm making progress so no one is hassling me. Good to develop a working relationship with the zoning and listers departments, however they exist where you are. Also recognize that there are probably politics involved, so stay sensitive to that.