One of our houserules for Castle Ravenloft is that a player gains full HP as part of leveling up. This was based on a common presumption between the "DM" (technically, CR has no DM per se, but he usually DMs our D&D home games) and players, that the same holds true in regular D&D.
However, after a quick review of the "Gaining Levels" section in PHB, I cannot find a rule that supports this assumption. I inquired of our DM, and he asserted that an extended rest is required to level up and therefore a restoration to full HP is implied.
I checked the PHB again, and was still unable to find such a rule. In fact, the only mention of timing for gaining levels is this:
Each time you overcome a noncombat encounter, defeat monsters, or complete a quest, your Dungeon master awards you experience points (XP). When you earn enough XP, you reach a new level.
Player's Handbook, p. 27
By this text, it would seem that no rest is required at all - you level up immediately upon acquiring the requisite XP. It looks like you should be able to (if you so choose) go from one encounter to the next without even so much as a short rest, and still level up in between.
In fact, by the quoted text alone (correct me if other rules apply) it could be construed that one may even be able to level up in the middle of a combat encounter. This would require that the DM awards XP after each kill - which I'm sure no one wants to bother with - but the "rules as written" seem to allow for it.
So, this question is really two-part:
- Does leveling up require any rest? If so, is it a short rest or an extended rest that is mandatory?
- With or without a mandatory rest, does a PC gain full hit points at the new level?