Can all creatures choose between advancing Hit Dice by type and advancing by class when leveling up?

Question

Many creatures advance solely by Hit Dice, while others are a mix of HD and class levels. Many PCs consist solely of class levels, but the Savage Species sourcebook introduced rules for playing and advancing creatures that were traditionally considered monstrous foes as PCs. Monsters were further broken down into "Hit Dice Levels". The Savage Progressions series of articles furthered this concept.

While in many cases a Hit Die level by creature type would be considered a sub-par leveling choice, can all creatures choose between advancing HD by type and advancing by class when leveling up?

This question was inspired by this question and the comments between HeyICanChan and Nijineko.

Answer 1

The following quotes indicate that it IS in fact a choice to level up by Hit Dice Type or by Class level, assuming that the prerequisites are met for the latter:

• "A creature’s Hit Dice total is also treated as its level....",
• "Often, intelligent creatures advance by gaining a level in a character class instead of just gaining a new Hit Die",
• "...options for altering standard monsters other than adding character classes or using the monster advancement...",
• "You must advance a monster by type if it lacks the Intelligence score to gain class levels (minimum 3).",
• "It gains an ability score increase for every four additional HD; it gains a new feat for every three additional HD; and it gains skill points for each additional HD."
• "In general, it is most advantageous for a 1 Hit Die creature with a +0 level adjustment to abandon its monster Hit Die and take class levels." ... "On the other hand, you may wish to keep that one monster Hit Die."

These quotes come from the Monster Manual 1 and Savage Species.

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This question was also raised when dragons were converted to a PC playable race. The following are supporting quotes from the Draconomicon dealing with the problem of aging (and thus gaining Hit Dice + dragon abilities) versus leveling up with class levels, and how to choose:

Advancement and Aging

• As it ages, as shown on Table 3–21: Aging for Dragon PCs, the dragon is required to devote a level every few years to its dragon “class,” reflecting the extra Hit Die or level adjustment it gains from aging. The character must add this dragon level as the first level it gains after reaching an age shown on the table. It gains no benefit from reaching a new age category until it attains this level.
• A dragon PC begins at a specified age (in accordance with the current party level in the campaign) and gains character levels as the player wishes over the course of its adventures.
• Once in a while, a dragon character must advance a dragon level without gaining a Hit Die, base attack bonus or save bonus increase, or any other tangible benefit. These levels are indicated on Table 3–21 by asterisks in the “Age in Years” column: When the dragon reaches the specified age, the next level the character attains must be used to advance its dragon level. When this occurs, its level adjustment increases (and its ECL rises accordingly) but the character receives no other benefits. These “extraneous levels” smooth the transitions between age categories, which are often accompanied by an increase in level adjustment to account for changes in size, ability scores, and other characteristics. In most cases on Table 3–21, a dragon PC that is close to reaching a new age category has a higher level adjustment than is specified for its age category and kind in the Monster Manual. This method of transitioning between age categories ensures that a dragon PC does not make rapid jumps in power out of proportion to its ECL by reaching a new age category and gaining the benefits of as many as three effective character levels at once....

This method presents a balanced approach based on the expectation of gaining more than one character level over the course of an in-game year of time. While the specific guidance is intended for dragons-as-player-characters, the concepts presented are excellent overall guidance-of-intent on choosing between advancing Hit Dice by type and advancing by class level when leveling up a character.

Answer 2

No, each option has creatures that cannot use it

There are creatures who can only advance by HD, and other creatures who can only advance by class level. Therefore, it is not true that all creatures can choose.

Player characters must “advance” by class level

Really, player characters don’t advance the way non-player characters do at all. Instead, they advance when they “gain a level” via XP, which is a different process.

Advancing a Level: When your character’s XP total reaches at least the minimum XP needed for a new character level (see Table 3–2), he or she “goes up a level.”

^{(Player’s Handbook, pg. 58)}

Leveling up always starts with choosing a class—and a class must be chosen, a non-class racial hit die is not an option:

When your character attains a new level, make these changes.

1. Choose Class: […]

^{(Player’s Handbook, pg. 58)}

This is not optional, and there is not an alternative.¹ Savage Species (and some other publications) provide “racial classes” or “template classes” to allow a player character to effectively “gain levels” in a powerful race or template, and levels in those classes may even involve racial hit dice or level adjustment, but they are first-and-foremost classes, which they must be in order to be chosen for step 1 of the level-up process. That they are classes designed to gradually replicate the features of a powerful race or template is, for this process, irrelevant.

1. _{As they so often are, dragons are an exception here. As nijineko’s answer demonstrates, under certain circumstances, a dragon PC not only can, but “is required to devote a level every few years to its dragon ‘class,’” per Draconomicon. Even here, though, this is treated as a kinda-sorta class level. It certainly doesn’t apply outside of dragon PCs who have reached a new age category.}

Creatures with low or no intelligence must advance by HD

Intelligent creatures that are reasonably humanoid in shape most commonly advance by adding class levels.

[…]

nonintelligent monsters, can advance by increasing their Hit Dice

^{(Monster Manual, pg. 290)}

a high enough Intelligence (minimum 3) to gain class levels

^{(Monster Manual, pg. 293; note this is in the Level Adjustment section, which is annoying since is the only mention of the Int 3 minimum in the core rules that I can find)}

Intelligent non-player creatures can probably do either

The rules say that intelligent, “reasonably humanoid in shape” creatures “most commonly” advance by class, but that doesn’t say they can’t advance by HD. The description of advancing by type says that that creatures that are “not humanoid in shape” or “nonintelligent” “can” advance by HD, which isn’t the same as “must.”

On the other hand, there’s an argument to be made that just because intelligent, humanoid creatures “most commonly” advance by class, doesn’t automatically mean they can choose to advance by HD. Nothing says they “can” do that, the way the rules say nonintelligent or non-humanoid creatures “can.” Likewise, nonintelligent or non-humanoid creatures “can” advance by HD, but nothing says they “can” alternatively advance by class.

Ultimately, though, these are DM-facing “rules” that are more like guidelines in the first place. Monster advancement is a very open-ended section of the rules, with lots of suggestions for DMs for how they can do things but still left pretty open. And if nothing else, there’s always rule 0.