I think a tiny house would be classified like a mobile home.
Mobile homes are required to be set up on a foundation by a HUD installer in st. marys county
All new homes require a sprinkler system in st. mary's county (including mobile homes)
mobile homes are taxable as houses because they are "fixated"
A tiny home would seemingly avoid all of these regulations. Mobile home manufacturers sell "tiny homes" called "park models" which is classified as as RV's (no more than 399 sq ft) which is enough reason for the county to say "hey you aren't allowed to live in this"
Campground and Day or Boarding Camp. 12 a. General Standards: 13 (1) Site plan approval shall be required. 14 (2) Year-round residency within tents and recreational vehicles in campgrounds is prohibited.
No mobile recreational vehicle or camping trailer may be occupied for more han three consecutive months.
https://www.stmarysmd.com/docs/CurrentZO.pdf
121. Recreational Vehicles. 6 a. Accessory Standards: 7 (1) Recreational vehicles may not be occupied as permanent residences.
8 (2) In allowable zones, the property owner, his tenants or guests may make use of one camping or recreational trailer on the owner's lot subject to the following conditions:
(a) Use shall be on a parcel of 1 acre or more.
(b) The camping or recreational trailer may only be used on an intermittent basis by the aforesaid individuals for private recreational use and enjoyment of the owner's property, such as for camping or hunting.
(c) No person shall be entitled to reside in the recreational vehicle for more than seven consecutive nights.
(d) The camping or recreational trailer must be completely screened from the view of dwellings on adjacent lots by a fence or evergreen buffer. 19
(e) Each camping or recreational trailer shall be equipped with an authorized self-contained sewerage holding tank. (3) Recreational vehicles shall comply with the floodplain provisions of this 22 Ordinance.
https://www.stmarysmd.com/docs/CZO.pdf