diff --git a/scenes/world/world.gd b/scenes/world/world.gd
index 09a5f86..1542287 100644
--- a/scenes/world/world.gd
+++ b/scenes/world/world.gd
@@ -642,6 +642,17 @@ var _build_sites_by_tile: Dictionary = {}
 
 
 func _on_designation_added(cell: Vector2i, tool: StringName) -> void:
+	# Replacement rule: painting a door on a tile occupied by a Wall (ghost or
+	# completed) demolishes the wall in place so the door build can proceed.
+	# Mirrors the Going-Medieval / Rimworld "door replaces wall" convention.
+	# Source of truth is World.build_queue — pre-built walls (test shed, cabin
+	# seed) aren't in _build_sites_by_tile but ARE in build_queue via _ready.
+	if tool == &"build_door":
+		var wall_at_cell = _find_wall_at(cell)
+		if wall_at_cell != null:
+			_demolish_wall_in_place(wall_at_cell, cell)
+			_build_sites_by_tile.erase(cell)  # may not be present; erase is no-op
+			# Fall through to spawn the door ghost below.
 	if _build_sites_by_tile.has(cell):
 		return  # already a build site here
 	# Phase 17 — read material override from the Designation controller (may be "").
@@ -747,6 +758,45 @@ func _on_designation_added(cell: Vector2i, tool: StringName) -> void:
 	Audit.log("world", "queued %s at %s" % [tool, cell])
 
 
+## True if `entity` is a Wall (ghost or completed). Duck-typed by script path
+## to avoid registration-order issues with class_name Wall.
+func _entity_is_wall(entity) -> bool:
+	if entity == null:
+		return false
+	var s = entity.get_script()
+	return s != null and s.resource_path.ends_with("/wall.gd")
+
+
+## Finds the Wall entity at `cell` (ghost or completed), or null. Searches
+## World.build_queue first since every Wall self-registers there on _ready;
+## that covers both designation-painted walls and pre-built _spawn_complete_wall
+## seeds.
+func _find_wall_at(cell: Vector2i):
+	for site in World.build_queue:
+		if not is_instance_valid(site):
+			continue
+		if site.get("tile") != cell:
+			continue
+		if _entity_is_wall(site):
+			return site
+	return null
+
+
+## Atomically remove a Wall entity at `cell`. Reverses Wall._complete() effects
+## if the wall was already built: unstamps the wall_layer TileMap, marks the
+## pathfinder cell walkable, and triggers a room recompute. Then frees the
+## entity. Used by the door-replaces-wall replacement rule.
+func _demolish_wall_in_place(wall, cell: Vector2i) -> void:
+	var was_completed: bool = wall.has_method("is_completed") and wall.is_completed()
+	if was_completed:
+		wall_layer.erase_cell(cell)
+		pathfinder.set_cell_walkable(cell, true)
+		if room_detector != null:
+			room_detector.recompute_around(cell)
+	wall.queue_free()
+	Audit.log("world", "wall at %s demolished (replaced by door designation)" % cell)
+
+
 ## Instantiate a Workbench ghost (in build-queue state) at `tile` with the
 ## given label and accepted skill. Returns the entity (already add_child'd).
 func _spawn_workbench(tile: Vector2i, label: String, skill: StringName):