Can you give racial HD to a 0 racial HD race?

[Reksew_Trebla]

So this is of course as the GM, but I just wanted to see if there was anything that forbid this before I do it.

I know if you add class levels to a creature with racial HD, they get a +4, +4, +2, +2, +0, -2 assigned to their ability scores in a manner that best benefits their class levels, but would that still apply if they started with class levels and gained racial HD later?

Also, would the general rule about increasing in size for increasing racial HD by 50% or more apply, meaning even a single racial HD increases 0 racial HD races' size?

general rule text:

As a general rule, creatures whose Hit Dice increase by 50% or more should also increase in size, but GMs should feel free to ignore this rule if warranted by the individual creature or situation.

I'm probably overthinking this, but I thought it would be a fun experiment to advance by racial HD for 0 racial HD races.

[Mysterious Stranger]

What is the point of this? If you are looking to create a special creature, just give it what you want. This seems to be a pointless experiment for no reason.

[Tim Emrick]

I think the justification for the ability score modifiers for adding class levels is that such creatures are exceptional members of their races to be able to acquire class levels instead of simply advancing in racial HD. (Similar to how PCs are exceptional members of 0-HD races that are usually limited to NPC classes.)

The only method that I know of for a 0-HD race to gain HD that aren't from a class is through the application of a template. And all of the templates that I can think of offhand that do this (mostly undead) have their own particular ability score adjustments.

If you want a creature with 0 racial HD to gain a size increase based on being bigger/tougher/older/etc., it is significantly less complicated to just apply the giant simple template. Heck, add it multiple times if you have to, to get the result you want.

[Belafon]

Forbid, no. I mean, you're the GM, after all.

They would not get any ability score bonuses since they are already class members. (Unless you give them 4HD - every creature gets +1 to a single ability score for every 4HD.) It sounds like you're going to be using the rules on Adding Racial Hit Dice from the Bestiary. Which, as the first sentence of that section says: "Adding racial Hit Dice to a monster is a similar process to building a monster from scratch."

I personally wouldn't bother. Class levels are going to be a better choice than what you get using just what's explicitly spelled out in that section of the Bestiary. Of course as the GM you do all kinds of additional things. Including giving new special abilities or other powers. (Which the section says you can do but with no "rules" for doing so.) So as Mysterious Stranger said, just give it what you want.

The whole section on creating and advancing Creatures advises over and over again "these are guidelines, the GM needs to both be creative and compare the power level to other creatures when using these sections."

[Reksew_Trebla]

Okay, I see I'm being a bit silly about this. I just thought it'd be interesting to use the Monster Advancement rules to make a 0 racial HD race have racial HD.

Thank you all for the responses.