Is it reasonable to build a sunny room with the permission of the property department?

The delivery area is only the yard or balcony area, but it is different if you seal it yourself. According to national laws, strictly speaking, it is indeed an illegal building. Generally, people like you, keep a low profile, don't go too far, and don't get too stiff with neighbors or property. If no one reports it, they generally don't come to check it.