I was doing some research in relation to this question, and I realized that I had run into a snag. I'm not aware of any rules in either D&D 3.5 or Pathfinder that directly relate to creating buildings underground.
I'm aware of the downtime rules, but the process to create buildings using these rules has a number of assumptions that make them non-helpful to me, namely that they assume that you're building in an existing town, and that there is no mention of how the rules would work underground.
I'm also aware of the Stronghold Builder's Guide, but I no longer have a copy, so I can't find out if there are specific rules about underground structures there.
Are there any rules for building structures in 3.5 or PF that specifically address building those structures underground?