Moveable doesn't mean portable, like trailers. It means more like sheds on a slab, not solid walls with footings and foundations and crawl space mechanicals. Sometimes it's just framing and steel sheet siding, no insulation. It's important to understand the type of building. They range from concrete block multi-level buildings to steel shed-looking garage rows with rollup doors (for car or secure storage) and even just open bays (RV and boat storage) and parking areas behind a fence. For instance, a client had one entire side of their building encroached on by a BP fueling station, and lost their easement right on that side, so the doors were removed and steel siding closed off that side. Record and document storage rented to businesses will be climate controlled and even have fire suppression systems. There may be an on site manager residence, card controller gates for self-entry. That's why a segregation study would be useful in a large facility. A lot of the older places are just leftover machine sheds. The industry is lucrative because of these tax treatments.

Your taxpayer's investment seems fairly low, so it's likely going to be some simple outbuilding shed type of structure.