diff --git a/src/AcDream.App/Rendering/GameWindow.cs b/src/AcDream.App/Rendering/GameWindow.cs
index 620c931..302a124 100644
--- a/src/AcDream.App/Rendering/GameWindow.cs
+++ b/src/AcDream.App/Rendering/GameWindow.cs
@@ -5741,15 +5741,12 @@ public sealed class GameWindow : IDisposable
                         * System.Numerics.Matrix4x4.CreateTranslation(bldOriginWorld);
 
                     // Derive the outdoor landcell id containing this building.
-                    // Retail's cell index: row-major (gridX * 8 + gridY + 1) within
-                    // the 8×8 grid of 24m cells in a landblock.
-                    int bldGridX = (int)(building.Frame.Origin.X / 24f);
-                    int bldGridY = (int)(building.Frame.Origin.Y / 24f);
-                    if (bldGridX < 0) bldGridX = 0;
-                    if (bldGridX >= 8) bldGridX = 7;
-                    if (bldGridY < 0) bldGridY = 0;
-                    if (bldGridY >= 8) bldGridY = 7;
-                    uint landcellLow = (uint)(bldGridX * 8 + bldGridY + 1);
+                    // Reuse TerrainSurface.ComputeOutdoorCellId rather than
+                    // re-deriving the row-major (gridX * 8 + gridY + 1) formula here.
+                    // Frame.Origin is landblock-relative, same coordinate space as
+                    // ComputeOutdoorCellId expects (local X/Y within the 192m block).
+                    uint landcellLow = terrainSurface.ComputeOutdoorCellId(
+                        building.Frame.Origin.X, building.Frame.Origin.Y);
                     uint landcellId = lbPrefix | landcellLow;
 
                     _physicsDataCache.CacheBuilding(landcellId, bldPortals, buildingTransform);
diff --git a/src/AcDream.Core/Physics/BuildingPhysics.cs b/src/AcDream.Core/Physics/BuildingPhysics.cs
index c05cd66..f717a58 100644
--- a/src/AcDream.Core/Physics/BuildingPhysics.cs
+++ b/src/AcDream.Core/Physics/BuildingPhysics.cs
@@ -1,5 +1,6 @@
 using System.Collections.Generic;
 using System.Numerics;
+using DatReaderWriter.Enums;
 
 namespace AcDream.Core.Physics;
 
@@ -36,6 +37,16 @@ public readonly struct BldPortalInfo
     public ushort OtherPortalId { get; }
     public ushort Flags         { get; }
 
-    /// <summary>Bit 0 of Flags (<c>PortalFlags.ExactMatch = 0x0001</c>).</summary>
-    public bool ExactMatch => (Flags & 0x0001) != 0;
+    /// <summary>
+    /// Bit 0 of <see cref="Flags"/> (<c>DatReaderWriter.Enums.PortalFlags.ExactMatch</c>).
+    ///
+    /// <para>
+    /// Reserved per retail's <c>CBldPortal::exact_match</c>. NOT currently
+    /// consumed by <see cref="CellTransit.CheckBuildingTransit"/> — every
+    /// portal overlap is treated as a valid entry trigger. If a future
+    /// regression surfaces (e.g., a building entered by overlapping a
+    /// non-exact-match portal), wire this into the entry test.
+    /// </para>
+    /// </summary>
+    public bool ExactMatch => (Flags & (ushort)PortalFlags.ExactMatch) != 0;
 }
diff --git a/src/AcDream.Core/Physics/CellTransit.cs b/src/AcDream.Core/Physics/CellTransit.cs
index 0e3e566..12bfa0d 100644
--- a/src/AcDream.Core/Physics/CellTransit.cs
+++ b/src/AcDream.Core/Physics/CellTransit.cs
@@ -158,6 +158,23 @@ public static class CellTransit
     /// whether the sphere center is inside it via
     /// <see cref="BSPQuery.PointInsideCellBsp"/>. If so, add the interior
     /// cell to <paramref name="candidates"/>.
+    ///
+    /// <para>
+    /// <b>Retail divergence:</b> retail's <c>check_building_transit</c>
+    /// uses <c>CCellStruct::sphere_intersects_cell</c> (radius-aware
+    /// BSP-vs-sphere test) which fires the moment ANY part of the sphere
+    /// overlaps the destination cell. Our port uses
+    /// <see cref="BSPQuery.PointInsideCellBsp"/> (radius-less, tests only
+    /// the sphere CENTER). Practical effect: entry into a building fires
+    /// when the player's foot-sphere center crosses the destination cell
+    /// boundary — roughly <paramref name="sphereRadius"/> (~0.48m) DEEPER
+    /// into the doorway than retail. If visual verification at the cottage
+    /// door shows a noticeable "late entry" effect (player visually inside
+    /// the building before walls switch from outdoor-stab to indoor-cell),
+    /// port <c>sphere_intersects_cell</c> in a follow-up.
+    /// <paramref name="sphereRadius"/> is plumbed through for that future
+    /// upgrade; currently unused.
+    /// </para>
     /// </summary>
     public static void CheckBuildingTransit(
         PhysicsDataCache cache,
diff --git a/tests/AcDream.Core.Tests/Physics/CellTransitCheckBuildingTransitTests.cs b/tests/AcDream.Core.Tests/Physics/CellTransitCheckBuildingTransitTests.cs
index e6cb512..6ea51dc 100644
--- a/tests/AcDream.Core.Tests/Physics/CellTransitCheckBuildingTransitTests.cs
+++ b/tests/AcDream.Core.Tests/Physics/CellTransitCheckBuildingTransitTests.cs
@@ -8,8 +8,17 @@ namespace AcDream.Core.Tests.Physics;
 public class CellTransitCheckBuildingTransitTests
 {
     [Fact]
-    public void SphereInsideBuildingPortalDestination_AddsInteriorCell()
+    public void BuildingPortalWithUnloadedCellBSP_NoCandidateAdded()
     {
+        // Verifies the null-CellBSP guard: when the destination interior cell
+        // is cached but its CellBSP isn't yet loaded (or is structurally absent),
+        // CheckBuildingTransit must NOT add the cell to candidates — even though
+        // PointInsideCellBsp(null, _) returns true.
+        //
+        // Happy-path (CellBSP present, sphere inside) requires a synthetic
+        // CellBSPTree which is non-trivial to construct from DatReaderWriter
+        // types. Deferred to visual verification.
+
         // Building at world origin. One portal to interior cell 0xA9B40100.
         var building = new BuildingPhysics
         {