diff --git a/src/interact/add_delete.rs b/src/interact/add_delete.rs
index 2db9b10..2ddd565 100644
--- a/src/interact/add_delete.rs
+++ b/src/interact/add_delete.rs
@@ -572,12 +572,11 @@ pub fn add_door_at_edge(
     }
 
     let corridor_cells = corridor_cells(layout, app.settings.cols, app.settings.rows);
-    let a_room = room_index_at_cell(&layout.rooms, edge.0);
-    let b_room = room_index_at_cell(&layout.rooms, edge.1);
     let a_in_corridor = corridor_cells.contains(&edge.0);
     let b_in_corridor = corridor_cells.contains(&edge.1);
+    let is_boundary = layout::is_room_boundary_edge(&layout.rooms, edge.0, edge.1);
 
-    if !manual_door_edge_allowed(a_room, b_room, a_in_corridor, b_in_corridor) {
+    if !manual_door_edge_allowed(is_boundary, a_in_corridor, b_in_corridor) {
         return false;
     }
 
@@ -597,18 +596,11 @@ pub fn add_door_at_edge(
 
 // Checks whether manual door edge allowed.
 pub fn manual_door_edge_allowed(
-    a_room: Option<usize>,
-    b_room: Option<usize>,
+    is_room_boundary: bool,
     a_in_corridor: bool,
     b_in_corridor: bool,
 ) -> bool {
-    if matches!((a_room, b_room), (Some(a), Some(b)) if a == b) {
-        return false;
-    }
-
-    let a_is_feature = a_room.is_some() || a_in_corridor;
-    let b_is_feature = b_room.is_some() || b_in_corridor;
-    a_is_feature || b_is_feature
+    is_room_boundary || a_in_corridor || b_in_corridor
 }
 
 // Draws add overlay.
diff --git a/src/layout/utils.rs b/src/layout/utils.rs
index e5cdf0a..b34c92c 100644
--- a/src/layout/utils.rs
+++ b/src/layout/utils.rs
@@ -201,6 +201,41 @@ pub fn shared_opening_width(a: &Room, b: &Room, span: usize, default_width: usiz
     default_width.max(1).min(span).min(max_width.max(1))
 }
 
+// Returns true if the edge between cell a and cell b is on the perimeter of at least one room.
+pub fn is_room_boundary_edge(rooms: &[Room], a: (usize, usize), b: (usize, usize)) -> bool {
+    for room in rooms {
+        let rx_start = room.x;
+        let rx_end = room.x + room.width;
+        let ry_start = room.y;
+        let ry_end = room.y + room.height;
+
+        // But since we have overlapping rooms, we check if this specific edge is one of the 4 perimeters of THIS room.
+
+        let horizontal = a.1 == b.1 && a.0.abs_diff(b.0) == 1;
+        let vertical = a.0 == b.0 && a.1.abs_diff(b.1) == 1;
+        if horizontal {
+            let left = a.0.min(b.0);
+            let right = a.0.max(b.0);
+            let y = a.1;
+            if y >= ry_start && y < ry_end {
+                if right == rx_start || left == rx_end - 1 {
+                    return true;
+                }
+            }
+        } else if vertical {
+            let top = a.1.min(b.1);
+            let bottom = a.1.max(b.1);
+            let x = a.0;
+            if x >= rx_start && x < rx_end {
+                if bottom == ry_start || top == ry_end - 1 {
+                    return true;
+                }
+            }
+        }
+    }
+    false
+}
+
 // Compute the shortest grid path between two cells using BFS.
 pub fn shortest_path_cells(
     start: (usize, usize),