RAW is it against the rules to take another class's class feature as a feat?

Question

For example, I want to play a Firbolg life cleric (because our party desperately needs more healing) but for roleplay purposes I don't want him to wear armor so I was thinking that I could take the Unarmored Defense Feat and use my Wisdom like a monk. So imagine my surprise when I found out that it wasn't a feat and just a class feature of some classes. Thus my question.

My DM is pretty is pretty lenient so as long as the rules don't specifically say "This is forbidden" she'll probably let me do it. Now I know that this could be easily solved by just taking 1 level in monk so that's an option but not my first.

There is an extremely related question, Gaining Other Class Feats, but that doesn't answer my "Is it explicitly forbidden?" question.

Other related questions (both from 3.5):

Is choosing a different class feature too good as a (house rule) replacement for a redundant feat? Answer seems to be "No, that is a fine rule."

What can I do with an existing feat that I later get free as a class feature? Answer is "Retrain the feat for a new one."

Answer 1

It is not "explicitly forbidden", but...

Class features are, like answered by indigochild in the question you linked, not feats. That's as explicit as the book gets: there's no direct statement saying that one cannot take a class feature as a feat, probably because they are two distinct things and the authors of the sourcebooks didn't think they would need to spell it out. Similarly, it's not explicitly forbidden to take 1000 hitpoints, extra spells, magic items, etc as feats, but as you can probably imagine, this kind of interpretation of RAW gets into very weird territory very fast.

Taking class features as if they were feats has a number of balance problems. Many of the class features offer extremely strong powers at an opportunity cost; one must take levels in the class one wants the powers of, and as a corollary, to gain the features of multiple classes one must multiclass, which trades out development in one's existing classes. Being able to cherry-pick class features as feats would allow "multiclassing without multiclassing", at the cost of a mere feat per class --- this would be totally broken.

Some class features offer benefits that can be received through feats. An example is the Martial Adept feat that allows one to take class features of the Fighter archetype Battlemaster. These are always an explicit part of the rules for the particular feat, not class features taken instead of a feat.

Answer 2

It's against the rules

You're basically asking "Can my character fire lasers out of their eyes because it doesn't specifically say it is forbidden?", to which the answer is, of course, no. *

There are no rules that allow you to pick and choose features from other classes as feats, feats are not related to class features and you're entirely in homebrew territory once you allow it. Simply going "there is no rule that disallows it" is ridiculous, because there's also no rule that says you can't write down "kills everything in 1 hit" on your character sheet. The game functions by rules and states what CAN be done, it doesn't try to list the 9 million weird combinations of things that can't be done.

* Unless your DM houserules that you are now the X-man Cyclops, in which case, go nuts.

Answer 3

I'm going to just go right out and say it: I don't like the wording of your question. I think it implies a bad precedent can have some dangerous (to game balance) implications.

"RAW is it against the rules to..." is not often followed by good intent. Not that I'm implying you have bad intent, merely an observed trend. The rules are both bounds for us to play within and also tools for us to enjoy the game through.

So, this answer is assuming best intent and that your question could/should be worded something to the effect of "Can I Use Feats to Increase AC without Using Armor?" or something similar.

Minor RAW answer

Magic Initiate can be used to gain 1/LR Mage Armor, which sets the target's AC equal to 13+Dexterity Modifier, as well as two cantrips! If that doesn't work for you and your game, then....

You're in homebrew territory

And that's fine. It may even be good; I love homebrew (but no IPAs). Have a discussion with your DM about your goals and visions.

As I talk about in this answer about a homebrew feat, you want to make sure that any new features follow a few rules:

1. Does this step on the toes of another player?
   • Don't make the party's Barbarian or Monk feel less special because your character doesn't like armor.
2. Is this overpowered and always going to get picked?
3. Is this fun / does it add anything?
4. Does this detract anything from the game?

In my own anecdotal observation, I feel that feats that are analogous to class features are less powerful (about half as powerful), such as Magic Initiate (half as good as a first-level sorcerer) or Martial Adept (less than half as good as Battle Master's Combat Superiority). With that in mind, you and your DM may decide that something like Unarmored Defense is the right direction, but too strong and may opt for a weaker version; perhaps 8+Dexterity+Wisdom, or for it to cap out at some value.

Answer 4

That's what multiclassing is for

Unlike getting a feat, in order to get a class feature you must have an appropriate level of the appropriate class. Some features are quite powerful and require a lot of levels (like cleric's Divine Intervention), some are accessible from the 1st level.

The Unarmored Defense feature is a cheap(-ish) one — you have to take only 1 level as a Monk the next time you level up. For more details see the PHB chapter 6 "Multiclassing".

Multiclassing for a caster is never really "cheap". Getting one less level of cleric and the spells related to that to pick up monk just for the AC is most likely a pretty poor tradeoff (thanks @Theik for this catch).