BJ, It's entirely possible to move most stick-built homes/additions, but of course there are limitations, which vary with each one. You'll have to make the determination of what possible problems you might incur, such as size of the building, distance and terrain over which it would travel. Most anything is possible...depending on how much you want it.
I once worked on a project where we moved a huge 100+ yr.old 2-story barn (in tact) over 40 miles and into suburban Kansas City so it could become part of a new home, but it meant having bridge railings, signs, stoplights, etc. removed & replaced. Special escorts and traffic control....all kinds of things; and it cost them a small fortune, but they wanted it bad enough.
When my mother relocated to our family farm, she decided to go with a modular home instead of building, but since this was set over a traditional concrete walk-out basement built into a hillside, it required us to bring in an 80-ton crane and crew from 200 miles away.
Try and keep these things in mind as you might design and build anything you plan to move later. If it looks like you'll run into major added costs and problems, you may want to go with the manufactured home addition instead. The best advice I can give you is to think this out as far ahead as possible, and weigh the pros & cons of each.