Stone extension
From Cantr II Wiki
A stone extension goes in a stone building or room and acts as a separate room, but doesn't cost as much or provide as much space as building another room would.
Construction
Object: Stone Extension
Days needed: 12
Raw materials needed: 10000 grams of stone
Objects: none
Tools needed: trowel
Capacity
People: 26
Weight: 3800000 grams