diff --git a/counterexamples/buildings/bad-floors-underground.yaml b/counterexamples/buildings/bad-floors-underground.yaml
new file mode 100644
index 00000000..be84741b
--- /dev/null
+++ b/counterexamples/buildings/bad-floors-underground.yaml
@@ -0,0 +1,15 @@
+---
+id: overture:buildings:footprint:1234
+type: Feature
+geometry:
+  type: Polygon
+  coordinates: [[[0, 0], [0, 1], [1, 1], [1, 0], [0, 0]]]
+properties:
+  theme: buildings
+  type: building
+  update_time: "2023-02-22T23:55:01-08:00"
+  version: 1
+  num_floors_underground: -1
+  # Expected errors:
+  ext_expected_errors:
+    - "exclusiveMinimum: got -1, want 0"
diff --git a/examples/buildings/building-polygon.yaml b/examples/buildings/building-polygon.yaml
index f6f2ca89..4872dcf1 100644
--- a/examples/buildings/building-polygon.yaml
+++ b/examples/buildings/building-polygon.yaml
@@ -22,6 +22,7 @@ properties:
   update_time: "2023-06-06T10:30:00-08:00"
   height: 21.34
   num_floors: 4
+  num_floors_underground: 1
   subtype: transportation
   class: parking
   is_underground: false
diff --git a/schema/buildings/defs.yaml b/schema/buildings/defs.yaml
index 798b7635..35d6448c 100644
--- a/schema/buildings/defs.yaml
+++ b/schema/buildings/defs.yaml
@@ -22,6 +22,11 @@ shapeContainer:
         Number of above-ground floors of the building or part.
       type: integer
       exclusiveMinimum: 0
+    num_floors_underground:
+      description: >-
+        Number of below-ground floors of the building or part.
+      type: integer
+      exclusiveMinimum: 0
     min_height:
       description: >-
         The height of the bottom part of building in meters. Used if a building or part of building starts above the ground level.