The one thing they will never make more of is land. Prices of lots to build on continue to escalate. So when people get all excited about the prospect of building a granny flat or an accessory dwelling unit it may well have to be in their own backyard. That can work well even if you add a secondary suite to the back of your home with a one level bath and bedroom. You can move your aging self in there and rent out the rest.
I will not get into zoning prohibitions here, because they vary so much. I highly suspect that as the age wave starts to crash against the shortcomings of real estate offerings, there will be some battle lines drawn as to what you can and can not do with your own property.
What if you only want a smaller home newly built with a reasonable Home Owners Association that does not tell you where you can put your flower pots but rather will cut the grass and remove snow? Enter the new real estate line of luxury villas. They come under the banner of luxury senior ‘Villas’ with price tags just a shade below what a reasonable small single family home used to cost but they are still row homes. They may be more than you want. Remember with production housing they always want to sell you want they have, any customization will cost more than an arm and a leg.
If you are retired in the western states you have a much better chance of finding more practical solutions. In an article called, Stashing Granny in the Backyard on Huff Post, I came across a builder than has expanded from the usual size single family homes or stacked stair challenged townhouse mode. They are offering a variety of plans for true homes versus what I call assisted living in the backyard suites.
Take a look at some of these smaller homes 600-1200 square feet that look like real homes. Imagine a group of them on a co housing site where the land has been bought in conjunction with several residents. They are called kit homes: https://www.pmhi.com/
Let me show you a few and think how much space would you need to not feel like you are living in a hat box.
This is the smallest at 600 square feet: https://www.pmhi.com/floorplans/granny-flats-600-1-200-ft.html
The larger one at 1196 square feet: https://www.pmhi.com/floorplans/granny-flats-600-1-200-ft/item/53-newport-granny-flat.html
I am not endorsing this product only featuring the idea that the real estate market may not give you what you want, but you can get what you need with some individual effort.