Yes for gps and aerial image, no problem. but ...
When I'm add information, I'm on the street, not with an aerial image ;) (I'm mapping with application CompleteStreet). And this app ask me how many floor on this building ? but it targets me balcon, or entry, or veranda or no apparent element :)
So I'm lost. Now, if I have a doubt and if I can delete element, I will think again and again :)

For roof level, I'm remember now but in both case, I think it was correct :-D (it's quite difficult to see level on last floor, we must walk away to correct see)
Undo all my problematics issue, like that it's clear and easier for you ! ;)

Oki for your information about Cadastre. For futur, I will check on josm and after go on the place to valid ! 8-)

Thanks