Ultimately it is up to your DM to decide if any rule will unbalance his game. I’ve been having a lively debate with my friendly neighborhood rules lawyer and I will throw in my input as well as my thought processes and references. (I must always provide her rulebook and page numbers. 🙂
I may paraphrase things unless I feel that phrasing is important then I will quote. You will have exactly where to look so you can read yourself to decide what is important. I will also try to explain my reasoning as to why certain rules exist and how they meld or clash.
So I will start with the rule in the Players Handbook (PH) that specific rules trump general rules (PH pg7) and the general rule for without armor is base AC = 10 + Dex mod.(PH pg14) If wear armor or carry a shield see chapter 5 . To me this makes sense. A standard 10 plus how quick you can get away from an incoming strike.
The next general rule is, “Some spells and class features give you a different way to calculate your AC. If you have multiple features that give you different ways calculate your AC, you choose which one to use.” (PH pg14) This rule is to prevent the AC stacking we have come to know and loathe. So you can now only choose either Mage Armor “The targets base AC becomes 13 + its Dexterity modifier.” (PH pg 256) or Unarmored Defense (seen below) for their “base AC”.
Since the debate is about the stacking of the Monks Unarmored Defense with the Druids Wild Shape ability let’s look at those rules which are specific rules.
The Monks Unarmored Defense says “… while you are wearing no armor and not wielding a shield, your AC = 10 + Dex mod + Wis mod.” (PH pg78) This is your new “base AC”. This class feature is definitely better than base AC without armor from above. The wisdom modifier is due to the training the monk has. This lets the monk use his insight to find a place where the incoming attack misses. (At least that’s my interpretation.) It’s a good day to be a monk it seems. You get the Dex mod and the Wis mod as long as you’re not wearing armor. “Wearing” is an operative word.
Wild Shape says, “Your game statistics are replaced by the statistics of the beast, but you retain your… Intelligence, Wisdom, and Charisma scores.” (PH pg67) It also says that, “You retain the benefit of any features from your class, race, or other source and can use them if the new form is physically capable of doing so.” So far this allows the Unarmored Defense features base AC equation from above and changes the base stats of the character to fit with the beast chosen.
Finally to this equation you add bonuses. Bonuses are benefits that be acquired by many means such as magic items, racial or class features, feats, etc. They are consistently signified in specific rules by the word “bonus”, ie: base weapon damage with a +2 bonus to damage. This means that there is something added to the base stat, ie: proficiency bonus added to skills, etc.
Many beasts have a “natural armor” bonus:
“A monster with a thick hide generally has a natural armor bonus of +1 to +3. The bonus can be higher if the creature is exceptionally well armored. A gorgon, for example, is covered in steely plates and has a natural armor bonus of +9.” (DMG pg276)
This additive effect is explained in the Dungeon Masters Guide (DMG). Insofar as AC is concerned it says, “A monster that doesn’t wear armor might have natural armor, in which case it has an AC equal to 10 + its Dexterity modifier + its natural armor bonus.” (DMG pg 276) Note that word “wear” in there folks.
So if you can keep your features (Unarmored Defense) and use the right stats for the situation then add your bonuses you have the following equation:
AC = Base AC + bonuses
So I just threw a bunch of stuff at you without any context so I will show you an example.
Elowin is a 2nd lv human moon druid and she wants to take her first level in monk. Her stats are (and this is no way optimized just so you know)
Her stats fall in the multiclassing rules (PH pg163-164) and she takes the plunge.
Now she has both Wild Shape and Unarmored Defense. Whatever shall we do? Since she wears armor she doesn’t even use Unarmored Defense until she shapeshifts. When she does she chooses a Brown Bear (PH pg304) absorbs her armor and gear as per the rules and takes its stats. Her new stats are:
So to determine our AC we look at our equation:
AC = 10 + Dex mod + Wis mod (our base AC) + bonuses
AC = 10 + 0 + 3 + 1 (natural armor bonus)
Elowin’s AC is now 14.
Not an overwhelming change from the bears base AC of 11 or the Unarmored Defense of 13. Just a touch more survivable.
A couple of caveats:
1. This is based on my understanding of the three basic books available now and doesn’t cover future expansions, house rules, etc.
2. Your GM has final say on all the rules including this one. Hey, the fact is if you’re playing with the GM you trust that you’re going to have a fair game. Unfair GMs end up with no players.
3. Everyone likes to optimize. Talk with your GM about what you want to do. State your case. You might be pleasantly surprised. The inverse of the above rule applies as well. Unfair players end up with no groups.
I hope this puts another perspective on the issue that helps.