That is because current rules say any elevation can only include habitable spaces that are adjacent to or above the adjacent grade level. (queensbp.org)
The owner must also declare that he will not be using this space as habitable space. (reimag.co.za)
They say they're safe for habitable spaces but 1) they produce an acrid and unpleasant smell while in operation which made us wonder if we were being poisoned, 2) they require two windows to be left slightly ajar to allow fumes / vapor to escape meaning that a lot of fuel is being burned just to keep the room at temperature, and 3) they produce copious amounts of vapor. (tinyhousebuild.com)