A Map Extension lets you add map content to an existing compiled map. For example, add new geometry to a map that ships with the game.
Map Extensions have the following limitations:
- It cannot modify anything in the parent map (Particularly: they cannot remove entities or affect baked lighting.)
- Static objects in the extension map won't cast shadows from static lights in the parent map.
- It can reference entities in the parent map via I/O, but must use the "final" fixed-up name rather than the Hammer name of the entity (use ent_text to find it).
First, create a new map in Hammer with File → New.
To have a good frame of reference, add a prefab that references an existing shipping map (find them in content/hlvr/maps/release/*)
IMPORTANT STEP: Select the prefab, expand the "Transform" group and check "Editor Only" - this lets you see the target map in Hammer but not include its content when you compile your map. (If compiling your map takes a long time, you may have forgotten this!)
Save and compile your map.
Re-run the game, and load the parent map using the 'addon_tools_map' command in the console:
You should see your extension map loaded and overlaid on top of the original map: