KidSnide
Adventurer
This is something I really don't want design space used on as far as rules and such as part of the main/base game.
I mean I can see the desire to have some nice handy reference for some of that kind of stuff, but every time you make a rule you kill a bit more of the freeform aspect of RP. I think the 4e theory of 'rules for resolving conflict only' is wise.
I think the key is that non-combat rules needs to be optional and properly tailored to the campaign. If a game is mostly about dungeon crawling, then castle building is just flavor text concerning what the PCs do in the background. There's no reason to have detailed rules about it. Whether or not the PC can build the castle is purely discretionary on the part of the GM.
However, if the game is Birthright or Kingmaker, then castle-building is the sort of thing that PCs are expected to do. In such a game, it is reasonable for the PCs to want to know how much a castle costs and what kind of benefit it provides in times of war. Tangible rules are necessary in such a game because a kingdom building player needs to be able to make informed decisions about where and when to build defenses. Completely generalized conflict resolution doesn't do that much good for such a game because the in-game decisions about constructions should (at least in some way) the in-game reality of that task.
That's why these sorts of rules should be intrinsically optional. They are only appropriate for certain types of campaigns. However, at the same time, they are important for the types of games that focus on those activities. With a collection of optional campaign rules, D&D can handle a much broader range of campaign styles.
-KS