It is often both. It is true that in many jurisdictions you cannot have a manufactured home, full stop. You also have minimum home size laws in residential sin

It is often both. It is true that in many jurisdictions you cannot have a manufactured home, full stop. You also have minimum home size laws in residential single family zoning areas, which either limit to one dwelling per lot (meaning no legal addition of an ADU when an existing home exists on the lot, no matter how it's built), or would not allow a stick built on concrete slab 300-350 square foot separate unit because the square footage is too small. Or because the local municipal laws do not allow for installation of a separate water supply, sewer service and electrical grid connection for a 2nd home on one lot. They only permit things like a detached garage which has a sub-panel attached to a home's primary 200A panel. So even if you solve the problem of it being not a "manufactured home" but legally a trailer you still can't drop it on a concrete slab or pilings and it has to legally remain a "RV trailer" with yearly vehicle registration/plate/tabs.