Do you have a copy of the foncier rate? That would probably be conclusive in a small case as to land and or “building”. .
The notary involved in the succession over the serviced plot should really confirm the position in relation to the immovable and the contraption as a whole.
History might help, as there is a specific depreciation régime for dismountable chalets put up for exhibition purposes, which are depreciable as movables. If the chalet is still dismountable, then it might qualify as a movable. The plot with services will however be immovable.
Caveat, the Eiffel Tower was dismountable, there are bolts to prove it but the idea seems to have become somewhat fixed.