Do HD or levels increase DCs for monsters?

[Idea Man]

I'm looking to increase the DCs of the special abilities of my critters. I believe hit dice added to amonster will increase the DC by one for every two HD added (confirmation requested). The only problem I have with that is that, past a certain point, the listings say I should enlarge my monsters, which I may not want to do. Could it be unbalancing in some way to not grow my monsters?

Also, I haven't found any mention of character levels adding to the DCs of special abilities. This makes leveling up monsters hurt in the "cool abilities" department. Could it be unbalancing to increase DCs when using class levels? (As an afterthought, I'd probably use a one-for-two ratio for related class levels and one-to-four for non-related levels.)

Obviously, increasing the attribute related to the ability will work. I haven't seen a Supernatural Ability Focus feat yet, but Spell-Like is fair game. If you have any other ideas, by all means.

[Titanium Dragon]

I'm looking to increase the DCs of the special abilities of my critters. I believe hit dice added to amonster will increase the DC by one for every two HD added (confirmation requested). The only problem I have with that is that, past a certain point, the listings say I should enlarge my monsters, which I may not want to do. Could it be unbalancing in some way to not grow my monsters?

Well, they won't be as hard; being large is, by and large, an advantage, so it doesn't quite scale properly - not to mention, size can give some indication of being harder, as well as, say, being more realistic that a 40 strength creature is huge rather than medium sized. That said, it wouldn't be the end of the world.

Also, I haven't found any mention of character levels adding to the DCs of special abilities. This makes leveling up monsters hurt in the "cool abilities" department. Could it be unbalancing to increase DCs when using class levels? (As an afterthought, I'd probably use a one-for-two ratio for related class levels and one-to-four for non-related levels.)

Obviously, increasing the attribute related to the ability will work. I haven't seen a Supernatural Ability Focus feat yet, but Spell-Like is fair game. If you have any other ideas, by all means.

Allegedly it is 1:1 for class levels, but I've found generally that ratio only works out on certain types of monsters and with certain classes at certain levels. A gnoll barbarian 5 is really a CR 6 challenge, but an adult white dragon which is also a 5th level fighter isn't really +5 CR harder than normal.

[RTGoodman]

I'm looking to increase the DCs of the special abilities of my critters.

In general, I believe a lot of abilities say if it is based on HD. This is especially true for many class-based abilities. If that's not enough, there's always the Ability Focus feat.

The only problem I have with that is that, past a certain point, the listings say I should enlarge my monsters, which I may not want to do. Could it be unbalancing in some way to not grow my monsters?

Well, larger size (as said above) is usually an advantage, since it gives a boost to Str and Con, increased Natural Armor, and improved reach. However, if you don't want them to get bigger, there's another option - as long as a creature has an Int of 3 or higher, it can take levels in a class, and those don't count towards the HD that increase size.

[tyckspoon]

I'm looking to increase the DCs of the special abilities of my critters. I believe hit dice added to amonster will increase the DC by one for every two HD added (confirmation requested). The only problem I have with that is that, past a certain point, the listings say I should enlarge my monsters, which I may not want to do. Could it be unbalancing in some way to not grow my monsters?

No, not really. As mentioned, that will tend to make them weaker most of the time- size provides extended reach, better grapple/disarm bonuses, and stat bonuses, which are fairly important for the melee-centric monsters that most typically get size increases from advancement.

Also, I haven't found any mention of character levels adding to the DCs of special abilities. This makes leveling up monsters hurt in the "cool abilities" department. Could it be unbalancing to increase DCs when using class levels? (As an afterthought, I'd probably use a one-for-two ratio for related class levels and one-to-four for non-related levels.)

Obviously, increasing the attribute related to the ability will work. I haven't seen a Supernatural Ability Focus feat yet, but Spell-Like is fair game. If you have any other ideas, by all means.

It's just called Ability Focus. It can be applied to any special attack form, the Su/Ex/Sp status is irrelevant. Character levels will usually not affect special ability DCs, that is correct- the formula is 10 + 1/2 racial HD + governing stat. Imbalance coming from letting class levels also apply to that would depend a lot on the specific monster and ability being buffed; for most abilities, it would probably be ok. The increase would theoretically just let the ability keep up with the DC it should have at whatever CR the monster winds up at anyway.

[AslanCross]

http://www.d20srd.org/srd/specialAbilities.htm

Spell-like abilities (Sp) mimic the DC they would have for the actual spell's level (like 3rd level for a fireball SLA, for example), plus the creature's key ability score. (Typically Charisma). So a creature with fireball as a spell-like ability with 18 charisma would have a DC of 17 on its ability. (10+3+4).

Supernatural (Su) ability DCs are calculated as such: 10+ (1/2 of creature's HD) + key ability score (typically Charisma).

The quickest way would to be to increase the key ability score and take the ability focus feat. Increasing the HD may not even work depending on the type, and might affect CR more (increasing size increases CR by one if you go by raw). Increasing HD tends to bloat the monster's AB and HP, I believe.

[Talic]

<Sp> and <Su> generally gain the Charisma Mod as a boost to DC, and <Ex> and Natural Abilities do vary somewhat, but generally gain the Constitution Modifier as a bonus (example: Natural Poisons).

[Fax Celestis]

You might want to read this thread's section on monsters. This section in the SRD has the information you're looking for:

Special Attacks and Special Qualities

Many creatures have unusual abilities. A monster entry breaks these abilities into special attacks and special qualities. The latter category includes defenses, vulnerabilities, and other special abilities that are not modes of attack. A special ability is either extraordinary (Ex), spell-like (Sp), or supernatural (Su). Additional information (when needed) is provided in the creature’s descriptive text.

When a special ability allows a saving throw, the kind of save and the save DC is noted in the descriptive text. Most saving throws against special abilities have DCs calculated as follows:

10 + ½ the attacker’s racial Hit Dice + the relevant ability modifier.

The save DC is given in the creature’s description along with the ability on which the DC is based.