I have a Tiefling Sorcerer who has Cold resistance from his race and also from his class (Sorcerer, Celestial bloodline: Celestial resistance, resist cold 5).
I can't find anywhere that says whether they stack or not. Of course I want them to stack, but the DM might not see it the same way :-)
Is there a definitive rule about this, or is it just going to be up to the DM?
For that matter, he is also an Inquisitor. He can also choose an Energy Resistance judgement....
Strictly speaking, I suspect they wouldn't stack; I probably wouldn't let either one stack with a Resist Energy spell, say.
Definitely not with a spell:
Energy Resistance: "A creature with resistance to energy has the ability (usually extraordinary) to ignore some damage of a certain type per attack, but it does not have total immunity.
Each resistance ability is defined by what energy type it resists and how many points of damage are resisted. It doesn't matter whether the damage has a mundane or magical source.
When resistance completely negates the damage from an energy attack, the attack does not disrupt a spell. This resistance does not stack with the resistance that a spell might provide."
The only specific restriction for stacking resistances is with spells. Specifically, Resist Energy/Protection from Energy do not stack with each other or with innate resistances.
1. This is a quantifiable value of benefit, and is not a bonus type. Such quantifiable benefits from multiple class/race sources usually stack, particularly abilities from multiple classes (See Sneak Attack, Uncanny Dodge). It's not the same as an armor bonus or enhancement bonus; such restricted bonuses always apply to a d20 roll or its target number (I'm sure there are exceptions, but nonetheless....).
2. A specific exception was made for spells. If they did not stack, the sentence would have been phrased "Energy Resistance from multiple sources, such as spells, does not stack."
3. In fact, it probably would sound something like this, substituting "Energy Resistance" for "Damage Reduction": "If a creature has damage reduction from more than one source, the two forms of damage reduction do not stack. Instead, the creature gets the benefit of the best damage reduction in a given situation."
It is, of course, possible that this was merely an oversight, as 3.5 did not allow nearly as many ways (that I'm aware of) to get energy resistance, so stacking never really came up.
However, I would view this like any cumulative bonus from classes and levels, such as base attack and saves.