Damn Flatlanders! Move north where the state isn't about to go down the budgetery toilet hole. Yes, a walkout would be great because you just need one well enforced room for safety. If two walls are already concreate the biggest expense is an above norm door frame and door. I plan on rolling outside and into the garage which is below ground level and has some very thick concrete walls because we're up a gentle slope from the water. We have an elevator but it does die with the electricity while once you know how the automatic garage door can be easily gotten around and opened. We get more thunderstorms and snow then tornados though. They do seem to be lighter weight ones by the time they get this far north anyway.
As others have pointed out do not depend on an architect or even the builder to not drop back to ADA guidelines if they have questions. I'd start a cork board of pictures of items or idea rooms that you like and a list of things you want, need and do plan on aging in place. And start the idea of a budget in your head.