Mark the room with the Mine order
Use Mine to mark the block of rock where you want the room. Choose the part you want to clear before marking it, so the excavation leaves space for your planned connection to the existing dungeon.
Let minions pick up the marked work
The official Malignus press kit describes an example where Mine marks rock east of the Great Hall and minions take the jobs themselves. Mark the work first and give your crew a chance to take it. If nobody starts, check whether Mining is allowed for the minions you expect to use and whether they have other priorities.
Build after the floor clears
Once the rock is gone, use the cleared area for construction. Start with enough space for the room you need, then decide whether to extend it after seeing how it connects to the rest of the dungeon.
FAQ
Which order starts a new room?
Use Mine to mark the rock you want excavated.
Do minions need to be assigned to every marked block?
The developer’s example says minions pull the marked mining jobs themselves.
Can I plan the room before construction?
Yes. Decide where you want the room and its connection to the dungeon, mark the rock, and build on the cleared floor.