Most uses of Diplomacy will probably use the version posted on Giant in the Playground Games
Brief summary: base DC = 15 + highest level in group + highest wisdom modifier in group.
Modifiers come in two flavors: relationship and risk/reward, both on a scale of -10 to +10. Relationship is for things like friendship, working for a common cause, personal enemies, people who are actively trying to steal from you, etc.. Risk/reward depends on what the likely outcome is, and how good (or bad) the results might be – from the perspective of the target. Asking a random stranger for something that doesn’t impose any cost on them or give any benefit means that the modifiers are both 0.
Limited use of PHB-style Diplomacy might occur, if it’s helpful. For instance, if the end goal really is “make the aristocrat your friend” rather than “connive him into letting you into his mansion”, then PHB-style diplomacy is