diff --git a/src/AcDream.Core/Physics/PhysicsEngine.cs b/src/AcDream.Core/Physics/PhysicsEngine.cs
index 29cdf9f..4f2b8dd 100644
--- a/src/AcDream.Core/Physics/PhysicsEngine.cs
+++ b/src/AcDream.Core/Physics/PhysicsEngine.cs
@@ -129,9 +129,20 @@ public sealed class PhysicsEngine
             targetCellId = physics.Terrain.ComputeOutdoorCellId(localCandX, localCandY);
         }
         else if (!currentlyIndoor && bestCellZ is not null
-                 && MathF.Abs(bestCellZ.Value - currentPos.Z) < stepUpHeight + 2f)
+                 && MathF.Abs(bestCellZ.Value - currentPos.Z) < stepUpHeight + 2f
+                 && bestCellZ.Value < terrainZ - 1f)
         {
             // Walked into an indoor cell from outdoor — transition to indoor.
+            // The extra guard `bestCellZ < terrainZ - 1` prevents transitioning
+            // into cells whose floor is AT or ABOVE terrain level — those are
+            // typically roofs, upper floors, or building footprints that overlap
+            // the outdoor terrain. A genuine indoor transition is into a cell
+            // whose floor is BELOW the terrain surface (basements, ground floors
+            // of buildings that sit on elevated terrain). Without this guard,
+            // standing near any building with a floor polygon covering the
+            // player's XY triggers an indoor transition and snaps Z to the
+            // cell's floor — which for multi-story buildings can be 30+ units
+            // below the outdoor terrain.
             targetZ = bestCellZ.Value;
             targetCellId = bestCell!.CellId & 0xFFFFu;
         }