Can you use the spell Alter self like this.

[Throne12]

Can you use the spell Alter self to change your skin to take on a rocky or metal like. Resulting in a higher ac.

[nickl_2000]

RAW? No. RAI: No

"When you cast the spell, choose one of the following options"
So your choices are what is in the spell.



RAF? Sounds like it could be cool, but it would be up to the DM
Shield of Faith is a first level concentration spell for 10 minutes and give +2
So, it seems about the same power level to give a +2 to AC with alter self since it lasts an hour and still requires concentration.

[BloodSnake'sCha]

Change Appearance. You transform your appearance. You decide what you look like, including your height, weight, facial features, sound of your voice, hair length, coloration, and distinguishing characteristics, if any. You can make yourself appear as a member of another race, though none of your statistics change. You also can't appear as a creature of a different size than you, and your basic shape stays the same; if you're bipedal, you can't use this spell to become quadrupedal, for instance. At any time for the duration of the spell, you can use your action to change your appearance in this way again.

I will say no by raw.

I recommend casting shield of faith instead.
Or barkskin
Or haste

If you look for AC


I will agree for it in my table but it is not RAW and you should not expect a DM you don't know to allow this.

[Stone-Ears]

The text of the spell reads

You assume a different form.
When you cast the spell, choose one of the following options, the effects of which last for the duration of the spell. While the spell lasts, you can end one option as an action to gain the benefits of a different one.

Aquatic Adaptation. You adapt your body to an aquatic environment, sprouting gills, and growing webbing between your fingers. You can breathe underwater and gain a swimming speed equal to your walking speed.
Change Appearance. You transform your appearance. You decide what you look like, including your height, weight, facial features, sound of your voice, hair length, coloration, and distinguishing characteristics, if any. You can make yourself appear as a member of another race, though none of your statistics change. You also don’t appear as a creature of a different size than you, and your basic shape stays the same; if you're bipedal, you can’t use this spell to become quadrupedal, for instance. At any time for the duration of the spell, you can use your action to change your appearance in this way again.
Natural Weapons. You grow claws, fangs, spines, horns, or a different natural weapon of your choice. Your unarmed strikes deal 1d6 bludgeoning, piercing, or slashing damage, as appropriate to the natural weapon you chose, and you are proficient with your unarmed strikes. Finally, the natural weapon is magic and you have a +1 bonus to the attack and damage rolls you make using it.

The only way your idea would work with Alter Self is with Natural Weapons, your rocky skin can make you hit harder and such. But I doubt that any but the most super lenient DMs would allow you to increase you AC with that idea.

[AHF]

RAW? No. RAI: No

"When you cast the spell, choose one of the following options"
So your choices are what is in the spell.



RAF? Sounds like it could be cool, but it would be up to the DM
Shield of Faith is a first level concentration spell for 10 minutes and give +2
So, it seems about the same power level to give a +2 to AC with alter self since it lasts an hour and still requires concentration.

I don't see how those are even close to the same power level if you are taking a spell that already does some neat things and then rolling another spell into it and expanding the duration of that rolled in spell by 6x. Alter Self already gives you the option to do 3 other things. As a DM, I'd let a player reskin a spell like Shield of Faith or Barkskin but wouldn't let someone add this to Alter Self.

[JNAProductions]

Yeah, the spell straight-up does not do this.

[Contrast]

Can you use the spell Alter self to change your skin to take on a rocky or metal like.

Sure.


Resulting in a higher ac.

No.

You can change your appearance to a rocky or metal like skin. Personally I would let this have minor mechanical effects (say you're in an area with acid fog or something that does minor environmental acid damage per round or something I may give you resistance). I would not allow it to boost AC though.

[Anymage]

If you really wanted to, I'd be okay letting it toughen your skin in a way that doesn't stack with armor. 13+dex mod, so you could essentially have mage armor with a shorter duration and that required concentration. A lesser version of a specific spell sounds about right for one function of a versatile spell.

Like everybody else is saying, letting it add to an existing AC is too much.

[Chronos]

Back in 3rd edition, natural armor was one of the many things that Alter Self could do. And it was one of the more overpowered spells. Which is why they toned it back in 5e.

I don't think we need to roll this one back.

[NecessaryWeevil]

If you really wanted to, I'd be okay letting it toughen your skin in a way that doesn't stack with armor. 13+dex mod, so you could essentially have mage armor with a shorter duration and that required concentration. A lesser version of a specific spell sounds about right for one function of a versatile spell.

Like everybody else is saying, letting it add to an existing AC is too much.

That seems like a reasonable houserule, about on par with the +1 natural weapon the book version gives you.