Despite the best efforts of reality TV shows to make "going tiny" look easy, the challenge remains for living legally
Finding the means to legally live in a tiny house is a hot topic for good reason:
Tiny houses make so much sense to those who choose to "go tiny," but restrictions on what defines a legal domicile often excludes tiny houses on wheels (or skids, blocks, and pontoons).
Of the myriad options, consider asking the county about any minimum square footage restrictions and other requirements that might inhibit your re-tasking something akin to a shed as the dwelling (akin = structural components meet "code minimum"). A well built shed might do the trick, though modular homes and small house kits can be purchased for quick assembly on-site within state mandated standards. Ask your local building official for direction on what they'll most likely pass versus immediately fail.
Another approach is to buy property with a dwelling in need of TLC, and fix it up over time. Buying a fixer-upper can be either a great investment or the proverbial "money pit." Be sure to have your budget and priorities in order, as buying property just to live in a shed might leave you with the headache of the property should you be booted for illegally camping. This is a touchy subject... trespassers beware, here there be monsters :)
Many rural counties seem open to using land for recreational purposes. I've not personally looked into buying property to setup a hunting camp, but it's been done. What's required for setting up a legal campsite varies widely, but may be an option so long as you're not planning to live there full-time. Should you get the okay for camping under some form of conditional use permit, a recreational vehicle might be required as they are purpose built as a (it's right there in the name) recreational vehicle. Currently, many tiny houses are self-classified as RVs, so hmm...
Similar in nature to the "live in a shed" approach, but on a larger scale, is building a garage apartment as part of a master plan for your property. In this way, you might be given approval to build a barn or garage with an apartment at the onset. In some cases, this may be allowable with the contingency of building the bigger home within a predetermined period of time. If you can have more than one habitable structure on the property, this might be a great way to start building something small and affordable, with the option to build something bigger later on (maybe by other future property owners).
If this approach is amenable to the county, be sure to site your garage/apartment (tiny house) where its appropriate for an outbuilding, leaving the best location for a larger more lavish structure. Building value into the little house on your property -- thinking of something like a classically designed cabin or timber frame -- provides inherent value you can leverage. Counties are more likely to say yes to a master plan than risk okaying a prospective eyesore that could cause rifts with your neighbors, be cited as a blight, and/or inhibit future property sale.
My thoughts are to do your due diligence in requests for legal living options alongside your soul searching about living in a tiny house. For some, a 400 square foot home seems tiny, and -- with the time, money, and energy -- is likely a reasonable alternative to living below the radar at the risk of getting kicked off your own property.
Here's hoping you find a way to unlock your tiny house dreams in the near future.