I looooooove a ranch. no stairs.

Master bedroom on the first floor. I've always loved it and the kids did fine. Baby monitors insure that you hear everything you need to hear (and more).

A lot has to do with how you design the stairs; if you do the code minimum, you have a (steep) 7 1/2" rise and a (short) 10" tread. Drop the rise to 7", or better yet 6", and extend the tread to 11 or 12 and you'll have a stair you can navigate into your 90's. And the exercise will do you good! Be sure to provide handrails on both sides, and make sure it's well lit for safety.
Often the better views are upstairs, so besides a gentle stair, I find that pre-planning a spot for a future elevator will make it a retreat you can live in forever. Just find a 4x5 foot closet space on all 2 or 3 levels that stack, and there's your elevator shaft! Good luck.

Downstairs.

I second every word that sfoody53 wrote (except the noise concern, which was never an issue for us).

My personal experience shows that having a livable unit on the first floor is indispensable, whether its the master suite or a spare/guest bedroom as long as full bathroom is also present on the floor (as opposed to a powder room most houses have). You plan to stay in the house for 30+ years. Think of your potential needs, not even 30 years down the line or when you age, but such examples as elderly parents or relatives coming to visit, leg injury to one of your family members, etc. Our downstairs suite had multiple reincarnations as circumstances demanded, and not having it may have prompted us to move on several occasions.

Consider a master 'suite' on both levels if possible.

Dy's suggestion for me is ideal. We have a master upstairs with our kids but created a guest suite downstairs that we could eventually use as a master if we either wanted more privacy or couldn't use the stairs. I also really like having guests not on the same floor as us and the kids (mine are still pretty little).

As a teen, I had the only downstairs bedroom in my parents house. It was off the den and close to the kitchen and of course there was a bathroom. This was very convenient when I had friends over.

My parents had an upstairs suite that included a huge music room that doubled as a den when they had company.

Flash forward several decades and my mother had a stroke which made going up the stairs impossible. She slept in my former bedroom. When my father passed away, my mother had to move in with my oldest sister.

I would aim for a master suite on ground floor and upstairs den or common area for kids, with at least one bedroom having a private full bathroom.

This situation gives parents and kids some privacy. Plus, it is becoming more common for an adult kid (even if married ) to live with parents a few years past college to save up for a down payment on a house. An upstairs master suite coupled with a den or common area would give them privacy.

Linda-I have the same dilemma. But if we add the master suite upstairs it would be towards the front of the house. While the kids' (6 and 8) rooms are currently downstairs. The kitchen would be between their bedrooms and the stairs. I'm worried if there was ever a kitchen fire I wouldn't be able to save my kids. Granted I am a worrier. But I saw a news show recently that showed kids sleep more soundly than adults and don't wake up to the sound of smoke detectors (even teenagers would sleep through the alarm).

I would like to just get away with paying for one room upstairs. But this fear makes me really consider putting all three rooms (master plus two kid rooms) upstairs. Ugh! The dilemma...

My sister built their forever home and installed an elevator to ensure they will have full use of the whole house. Something to consider.

We moved to a eusonian style open floor plan when our 4 kids were 4th grade to 9th grade. Master was on 3rd level, family room down 10 steps, then 4 Br, kitchen, rec room, laundry, another entrance on lowest level. We liked it, but good friends worried abt kids on lowest level. First night after move, 9 yr old had no trouble finding us in middle of night for something. Home was a delight for almost 20 years. We loved it. If we'd had babies, ababy monitor would have worked.

My husband and I chose to have our master downstairs when we built our home 14 years ago. We also put utility room downstairs. We wanted everything that we would need as we get older on the first level. My kids own entire upstairs of three bedrooms and full bath. I go days without going up there. We designed a laundry shoot that the kids open the door and throw laundry in and the laundry lands in my utility room.

Downstairs! And do the laundry shoot; you won't regret it!

Downstairs, no question. You will not be forced out of your home due to age or mobility issues. Even though you plan to stay in the house for a long time it is also better for re-sale.

We put our master on the first floor when we built 25 years ago. Our children were small, but they loved being upstairs. Now we close off the upstairs until our family comes home. My advice would be to put two bedrooms downstairs. It makes a great place to go if someone snores and also if an older parent visits it keeps them from having to climb the stairs.

building my dream house!