Under the SMAUG system currently used by PLANES, there is no player-building, however there is a very rich building system for immortals. Under EmpireMUD, there is a simple system to allow players to build a few dozen stock building types, using resources which they have gathered. This article
EmpireMUD Building Edit
Building in EmpireMUD is handled by players claiming a particular plot of land, then plopping down a building on that plot of land. Some buildings take four, nine, or even 16 plots of land.
The EmpireMUD system has an essential flaw, in that to build a great empire or base, you need to spend a great deal of time collecting resources. Because so much time is needed to collect the resources, and the goal for most players is to build a grand empire with all the amenities and defenses possible, the vast majority of time is spent simply botting or triggering for resources. This is categorically lame.
A Lucid Dreamer Building (speculative) Edit
Now, this is completely wide-open, but at first I was thinking of having an assess command to locate raw building materials in the wilderness, to fall in line with the other resource-location commands. That might get a bit cumbersome and could also lend itself to botting.
A totally different way that might work could be to buy the resources from, for instance, a lumberyard, quarry, etc. Perhaps being able to buy or build a lumberyard or quarry could be an option as well. At that point you're getting in to a sort of Real-Time-Strategy thing, though, which I'd kind of like to avoid. At any rate we're trying to promote interaction between characters (both PC-PC and PC-NPC) so those types of interactions should be rewarding to allow players to buy resources. For example, rewards from quests that require multiple players would be a great way to get player-characters to interact with NPC's and other player-characters. Rewards might include land claims and resource vouchers.