@openstreetmap/id-tagging-schema/data/fields/building/levels/underground.json

The OpenStreetMap tagging model used by the iD editor

{
    "key": "building:levels:underground",
    "type": "number",
    "minValue": 0,
    "label": "Underground Levels",
    "placeholder": "2, 4, 6...",
    "terms": [
        "basement levels"
    ]
}