Are there rules for building underground structures?

Question

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?

Answer 1

You are correct in assuming the stronghold builder's guide contains the information you are looking for. It has information on the costs of building below ground, including successive costs for deeper layers, and the difference between turning a cave of appropriate size and shape vs new excavations.

It costs an extra 10% to create an underground structure, though any hewn stone walls needed are free. There is an additional surcharge for each room built below the first subterranean layer, which is detailed on page 33 of the Stronghold Builder's Guide.

The Stronghold Builder's guide is 3.0, but with the exception of a few spell effects for wondrous architecture, is still the best official source.

Answer 2

Ultimate Rulership by Legendary Games is designed as an expansion to the kingdom building rules in Pathfinder's Ultimate Campaign. It has a section on building a "Cavern City", which is the closest thing I know about.

A cavern city is one built underground. Common among dwarves, drow, and similar deep dwellers, surface kingdoms can establish cavern cities as well. In some cases they resemble cities on the surface, constructed within massive vaulted caverns, often surrounded by fungus farms or smaller satellite quarries or mines. Other cavern cities, however, are simply interconnected cave complexes, wherein each city lot comprises its own warren of caves and chambers, linked by tunnels and passages to adjacent parts of the city.

It then follows with a list of changes to make to the standard rules in order to simulate building underground, one of which is the requirement to build a certain number of Tunnels (a new building type in the book) per number of regular lots you fill.

All in all, it takes up slightly less than one column of the book. It's good for adding an underground city to an existing kingdom (or starting a kingdom there), but only if you're already committed to using Pathfinder's rules in the first place. It doesn't model individual buildings very well, because that's too granular for the base system.