It would be perfect to find a master on main with other space upstairs so you can ignore that upstairs when company is not there.
My BIL has the casita arrangement @ChoatieMom talked about, and it’s great. A place for people to stay bu not part of the daily upkeep.
^That’s what my folks did. Master is on the main floor with guest bedrooms and bath upstairs. They don’t really go upstairs much as everything they need is on the main floor but the extra space is available when family or other guests visit and is also good for resale value when the time comes. They also built the house with a walk-in shower and doorways that are wide enough to accommodate a wheelchair should it ever be needed. I appreciate their forethought.
I wouldn’t buy a place where you would have to navigate stairs. Keep looking.
My in-laws seemed perfectly content navigating the stairs to their second floor bedroom when they were 80. Until one night FIL fell down the stairs and died a few hours later. Until then I had thought the concern about aging and stairs was somewhat hypothetical.
Another way to think of your choice - if you are planning to buy rather than rent then a place with a first floor master might have more resale value as our population ages.
My SisIL loved her townhouse in SF. The main living area was up a flight of stairs from the street, but it had a bedroom right as you entered as well. She was fine in it until she got cancer around age 70. Stairs got increasingly painful for her and she was mostly housebound after that, as stairs were just too tough. We love our one-story home. Our D’s friend who just got her degree in Occupational Therapy said our home is great for aging in place, though some of the thresholds have to be watched and the doorways are narrow for wheelchairs and walkers. At least it’s all one level and there’s only one step up from the street to the front door.
No stairs with a basement you can finish off as a carpeted rec room with futon and/or sofa beds. Basement can be used/heated/cooled when kids visit, and completely ignored the rest of the year. Grandkids can sleep down there or kids.
I don’t see how 2000 sq ft on the main floor would not be plenty of space for 2 people when set up as an open plan. Kitchen with living/dining room combo is my favorite setup. 2 or 3 bedrooms. 2 baths (1 master and 1 full bath used for guests and as powder room).
Absolutely no tri-level space!
To plan even further… even one story homes sometimes come with a couple of steps to the front door. Sure, one can build a ramp for the wheelechair, but is it going to cause an issue with the HOA? Also, if you are planning for all cases of limited mobility, make sure that the doorways and hallways are wide enough for a wheelchair, and there are sturdy chair rails for the wheelchair operator to bump into without crashing the sheetrock (yup, a big issue for one of the folks I know - his 95 yr old mom still living on her own got a motorized wheelchair and literally destroyed the walls).
Also, bathrooms that are built to support grab bars - whether from the start or added later when/if needed.
I can see I will have to rethink stairs!
My brother put in a stair lift in his house for the grandmas. Solved the steps issue for both that are living with him now. You have to have a regular width staircase. Straight is easier but a landing is not undoable, just more expensive. The seat tilts up out of the way when not in use so others can walk up the stairs.
Stairs are fine until they aren’t. My maternal grandmother was in her home into her 90s; the only bathroom was on the second floor. My mom was in great shape until her early eighties; she’s been in a wheel chair for five years despite always maintaining a healthy weight, doing a lot of walking and “heavy” gardening, etc. There’s just no way to know.
We are weeks away from moving into our downsize. It is 1800 SF on the main floor, which has everything we need if we have trouble with stairs in the future. Downstairs (the lot slopes to the rear) are a family room, guest rooms and baths, laundry and a sauna (our splurge). I live in a college town and will use the guest rooms for B&B during football and “event weekends”, for visiting family and friends at others.
I can’t imagine not having a yard to work in but am being mindful with the landscaping so I can turn it over to a service if I have to or if we are traveling for extended periods of time. Speaking of travel, we also have a teardrop trailer and are not above putting overflow (family) guests there-- http://littleguytrailers.com/tab/tb-cs-2/
Grab bars don’t have to look like nursing home equipment. We put in one our walk-in shower called the “Choreograph” shower barre by Kohler; it’s not bad looking at all. (It’s easiest if you can plan ahead and get the blocking in the wall when you’re building or remodeling.)
Currently in a 3 BR 2 full Bath (large family room and a bonus room) where we raised our child - looking to go no smaller than 2 BR and 1.5 bath and no family room (only living room). I would still do stairs at this point (in my 50’s) and move again if stairs are an issue.
DH and I had talked quite a bit about downsizing, and a one level home half the size of the current house (2,600 sf) would have been perfect for us. I once loved doing yard work but, since DH died last year, I find it’s become more of a chore. I would be very happy in a one level home with two bedrooms, two baths, and a one car garage. I’d like something where the yard work is included in HOA fees, but I do not want to leave my surrounding area. There is a Del Webb development being built about 20-25 minutes south of me, but that area is extremely congested due to accelerated growth and poor infrastructure planning, so that is pretty much out. I guess I’m staying put. : /
My in-laws are in a 3/2, one-level patio home with around 2,200 - 2,300 sq ft. Their yard is the size of a postage stamp. Are patio homes not an option where you are @snowball? I suppose in densely populated areas, land might be too valuable. They moved in fifteen years ago when in their mid-60’s but also had the foresight to put in wide hallways and doors for wheelchair accessibility. Still don’t need them, which is a blessing. They do have a large dining table. It’s interesting to me that is so important to your dh - particularly since it is from your side of the family.
Our criteria are similar to @thumper1 ‘s. We own a one-level, 935 sq ft, 2/2 condo in Florida on the ground floor. I do understand not necessarily wanting shared walls, however. That is our retirement plan. We went from a 4,300 to 3,400 sq ft home about five years ago, but just sold that home and are “practicing” small scale living by renting an apartment the same size as our condo. We have quickly and happily adjusted. Now my in-laws’ place seems huge to me! Walking the dog IS a hassle. We are not sure about future dog plans but have agreed to go “dogless” for one year after our current (pretty old) dog passes. We want to have the flexibility to lock and go. Not possible with a pooch. We only have one ds who will be a college senior next year. No gf in sight, so we are likely a ways from any grandchildren. But, our plan is to rent something for ds and his family when our space constraints become an issue. Not sure if this will work long-term for us, but we want to give it a go. Second bedroom will be multi-purpose (office, music, exercise) with a Murphy bed. As others have said, we don’t want to maintain space that we may only need two or three times a year. Even in our most recent home, dh and I rarely used more than four rooms.
So, I would encourage ditching the table and going even smaller. Maybe 1,800 sq ft?
We have figured, at the moment, the comfort size is about 1800-2000 square feet; not so much about the numbers, but what you get in our area in that size. The reason for 3 bedrooms is one will be a home office/guest bedroom as my husband and I both do a good bit of work at home; two bedrooms would not work, nor are they even available other than in a mid or high rise.
We enjoy having family and friends over for dinners, so a roomy kitchen is a must, along with a decent size family room for guest to congregate. The units with massive master with a sitting area is wasted space to us; are we going to entertain in our bedroom? 
I want my square footage in living area as that is where we are when home except when showering and sleeping.
At the moment, we are thinking this will not be our forever home, so we will select a place that works for us now; maybe it will work forever, who knows!
We started from scratch with an architect because we felt the way most homes are laid out would not be optimal for DH and me and how we use space. Hoping we got it right.
If you can limit the stairs to a bedroom upstairs you should be OK. I would never buy a home with steep stairs up from parking/garage to the main living area. That will become a huge chore within a month (and I’ve just sold 2 houses with this exact problem)
Go put some heavy bricks in some shopping bags and go up and down a set of stairs as practice. Furniture, groceries, cases of water or beer, library books, workout gear, dogs etc. Not to mention any construction stuff. No thanks!!! I’m getting older just thinking about it:)