diff --git a/data/scenarios/Testing/1780-structure-merge-expansion/sequential-placement.yaml b/data/scenarios/Testing/1780-structure-merge-expansion/sequential-placement.yaml
index c82fbb2c8..fc6660bad 100644
--- a/data/scenarios/Testing/1780-structure-merge-expansion/sequential-placement.yaml
+++ b/data/scenarios/Testing/1780-structure-merge-expansion/sequential-placement.yaml
@@ -28,11 +28,18 @@ world:
           - src: house
             orient:
               flip: true
-            offset: [-8, 0]
+            offset: [-6, 0]
           - src: house
+
+          # - src: house
+          #   orient:
+          #     flip: true
+          # - src: house
+          #   offset: [6, 0]
         map: "xx"
   palette:
     'Ω': [grass, erase, base]
+  mask: '.'
   placements:
     - src: block
       offset: [0, -1]
diff --git a/src/swarm-topography/Swarm/Game/Scenario/Topography/Structure/Assembly.hs b/src/swarm-topography/Swarm/Game/Scenario/Topography/Structure/Assembly.hs
index 7635e5669..769359e20 100644
--- a/src/swarm-topography/Swarm/Game/Scenario/Topography/Structure/Assembly.hs
+++ b/src/swarm-topography/Swarm/Game/Scenario/Topography/Structure/Assembly.hs
@@ -23,7 +23,6 @@ import Data.Text qualified as T
 import Linear.Affine
 import Swarm.Game.Location
 import Swarm.Game.Scenario.Topography.Area
-import Swarm.Game.Scenario.Topography.Grid
 import Swarm.Game.Scenario.Topography.Navigation.Waypoint
 import Swarm.Game.Scenario.Topography.Placement
 import Swarm.Game.Scenario.Topography.Structure
@@ -50,8 +49,7 @@ overlaySingleStructure
 
     let mergedWaypoints = inputWaypoints <> map (fmap $ placeOnArea overlayArea) overlayWaypoints
         mergedPlacements = inputPlacements <> map (placeOnArea overlayArea) overlayPlacements
-        inputGridContent = gridContent inputArea
-        mergedArea2 = overlayGridExpanded (show sName) inputGridContent pose overlayArea
+        mergedArea2 = overlayGridExpanded (show sName) inputArea pose overlayArea
         mergedArea = trace (unwords [
             "For placement of"
           , show sName
@@ -110,20 +108,35 @@ mergeStructures inheritedStrucDefs parentPlacement (Structure origArea subStruct
 
 overlayGridExpanded ::
   String -> 
-  Grid (Maybe a) ->
+  PositionedGrid (Maybe a) ->
   Pose ->
   PositionedGrid (Maybe a) ->
   PositionedGrid (Maybe a)
 overlayGridExpanded
   note
-  inputGrid
-  (Pose loc orientation)
-  (PositionedGrid c overlayArea) =
-    trace (unwords ["In overlayGridExpanded for", note, "where c = ", show c, "and loc = ", show loc]) output
+  baseGrid
+  (Pose yamlPlacementOffset orientation)
+  -- NOTE: The 'childAdjustedOrigin' is the sum of origin adjustments
+  -- to completely assemble some substructure. However, we discard
+  -- this when we place a substructure into a new base grid.
+  (PositionedGrid childAdjustedOrigin overlayArea) =
+    trace (unwords [
+        "In overlayGridExpanded for"
+      , note
+      , "where childAdjustedOrigin ="
+      , show childAdjustedOrigin
+      , "and placementOffset ="
+      , show yamlPlacementOffset
+      , "and placementAdjustedByOrigin ="
+      , show placementAdjustedByOrigin
+      , ". New origin will be:"
+      , show $ gridPosition output
+      ]) output
    where
-    output = PositionedGrid origin inputGrid <> positionedOverlay
+    output = baseGrid <> positionedOverlay
     reorientedOverlayCells = applyOrientationTransform orientation overlayArea
-    positionedOverlay = PositionedGrid loc reorientedOverlayCells
+    placementAdjustedByOrigin = gridPosition baseGrid .+^ (yamlPlacementOffset .-. origin)
+    positionedOverlay = PositionedGrid placementAdjustedByOrigin reorientedOverlayCells
 
 -- * Validation
 
diff --git a/src/swarm-topography/Swarm/Game/Scenario/Topography/Structure/Overlay.hs b/src/swarm-topography/Swarm/Game/Scenario/Topography/Structure/Overlay.hs
index 86444ed5a..0bff92854 100644
--- a/src/swarm-topography/Swarm/Game/Scenario/Topography/Structure/Overlay.hs
+++ b/src/swarm-topography/Swarm/Game/Scenario/Topography/Structure/Overlay.hs
@@ -9,10 +9,11 @@ module Swarm.Game.Scenario.Topography.Structure.Overlay (
 ) where
 
 import Control.Applicative
+import Debug.Trace
 import Data.Function (on)
 import Data.Int (Int32)
 import Data.Tuple (swap)
-import Linear
+import Linear hiding (trace)
 import Swarm.Game.Location
 import Swarm.Game.Scenario.Topography.Area
 import Swarm.Game.Scenario.Topography.Grid
@@ -113,10 +114,19 @@ instance (Alternative f) => Semigroup (PositionedGrid (f a)) where
     -- We don't have to adjust the origin if the base layer lies
     -- to the northwest of the overlay layer.
     clampedDelta = V2 (min 0 deltaX) (max 0 deltaY)
-    newOrigin = baseLoc .-^ clampedDelta
+    newOrigin2 = baseLoc .-^ clampedDelta
+
+    newOrigin = trace (unwords [
+        "COMBINING baseLoc"
+      , show baseLoc
+      , "with overlayLoc"
+      , show overlayLoc
+      , "to get"
+      , show newOrigin2
+      ]) newOrigin2
 
     paddedOverlayPair =
-      padSouthwest originDelta $
+      padSouthwest (overlayLoc .-. origin) $
         OverlayPair baseGrid overlayGrid
 
 -- | NOTE: We only make explicit grid adjustments for