diff --git a/app/layout/project/sidebar/project/inventory-panel/cost-surfaces/index.tsx b/app/layout/project/sidebar/project/inventory-panel/cost-surfaces/index.tsx
index 6b1404cc30..ee70f2c8d8 100644
--- a/app/layout/project/sidebar/project/inventory-panel/cost-surfaces/index.tsx
+++ b/app/layout/project/sidebar/project/inventory-panel/cost-surfaces/index.tsx
@@ -89,9 +89,13 @@ const InventoryPanelCostSurface = ({ noData: noDataMessage }: { noData: string }
 
   const handleSelectAll = useCallback(
     (evt: ChangeEvent<HTMLInputElement>) => {
-      setSelectedCostSurfaceIds(evt.target.checked ? costSurfaceIds : []);
+      setSelectedCostSurfaceIds(
+        evt.target.checked
+          ? filteredData?.filter(({ isCustom }) => isCustom).map((cs) => cs.id)
+          : []
+      );
     },
-    [costSurfaceIds]
+    [filteredData]
   );
 
   const handleSelectCostSurface = useCallback((evt: ChangeEvent<HTMLInputElement>) => {
diff --git a/app/layout/project/sidebar/project/inventory-panel/cost-surfaces/modals/delete/index.tsx b/app/layout/project/sidebar/project/inventory-panel/cost-surfaces/modals/delete/index.tsx
index f7a60e339a..7230cec00c 100644
--- a/app/layout/project/sidebar/project/inventory-panel/cost-surfaces/modals/delete/index.tsx
+++ b/app/layout/project/sidebar/project/inventory-panel/cost-surfaces/modals/delete/index.tsx
@@ -34,8 +34,9 @@ const DeleteModal = ({
 
   const selectedCostSurfaces = useMemo(() => {
     return (
-      allProjectCostSurfacesQuery.data?.filter(({ id }) => selectedCostSurfacesIds.includes(id)) ||
-      []
+      allProjectCostSurfacesQuery.data?.filter(
+        ({ id, isDefault }) => selectedCostSurfacesIds.includes(id) && !isDefault
+      ) || []
     );
   }, [allProjectCostSurfacesQuery.data, selectedCostSurfacesIds]);