When using the Craft skill to create armor. It says the difficulty to create an armor is 10+AC bonus. So, can I create an armor that gives 25 AC? The DC would be 35. I wonder if it's possible to create only existing armor. How does it work?
Ultimately, you can craft anything with the DMs permission, and nothing without it. Most games assume the printed armors are available and others are not, but you can always ask your DM to homebrew a 25-AC armor and let you craft it. I doubt he will; I wouldn’t. But he might, (probably) doesn’t hurt to ask.
What does hurt is actually making it.
The DC would be 35, not 25, and considering that 25 AC would be massive, the armor would have to be phenomenally expensive. Consider the existing armors:
- Scale Mail, 4 AC, 50 gp
- Banded Mail, 6 AC, 250 gp
- Full-plate, 8 AC, 1,500 gp
You see how quickly that ramps up? +2 AC quintuples or more the cost. You want to add 17 AC? That’s 58½ the cost of full-plate: over 1,000,000,000 gp. That’s a billion. Obviously, it’s not actually worth that, but what if it was just a hundred thousand, there are actually items worth 100,000 gp or more in the game.
A mundane item that cost a hundred grand, that is, a million silver pieces, with DC 35 that you hit precisely (i.e. taking-10 with a +25 bonus), takes 817 weeks to make. That’s 15 years of working eight-hours-a-day, day-in and day-out. Good luck.
No Guidelines Exist
If using the skill Craft (armorsmithing) to create a new kind of previously nonexistent armor,1 I suggest the craftsman work with the the DM to determine the new armor's statistics using existing armors as guidelines, just like what's supposed to happen with custom magic items.
If this isn't done--for example, if one's extrapolating based on a purely theoretical grounds or one's DM's a goldfish--, one's likely to end up in ridiculous places, with mundane armor possessing unfair statistics.2 It's unlikely beyond level 9 that this would be a serious problem--mid- and high-level characters are usually less concerned with their Armor Classes than they are with, for example, their saving throws--, but at lower levels Armor Class Infinity might be a cause for concern.
For comparison, the beefiest non-setting-specific armor is the heavy armor Mechanus plate (Planar Handbook 69, 70) (1,750 gp; 75 lbs.). The wearer gains a +10 armor bonus to Armor Class, and it possesses a Maximum Dexterity Bonus of +0, an Armor Check Penalty of -10, and Arcane Spell Failure 50%. It reduces the speed of a creature whose normal speed is 30 ft. to 15 ft. and the speed of a creature whose normal speed is 20 ft. to 10 ft.
The beefiest setting-specific armor is the Forgotten Realms' heavy armor Thaalud stone armor (Anauroch 108) (2,800 gp; 180 lbs.). The wearer gains a +12 armor bonus to Armor Class, and it possesses a Maximum Dexterity Bonus of +0, an Armor Check Penalty of -8, and Arcane Spell Failure 40%. It reduces the speed of a creature whose normal speed is 30 ft. to 20 ft. and the speed of a creature whose normal speed is 20 ft. to 15 ft.
Any masterwork armor can have its armor bonus increased by 1 with the property reinforced (Dragon #358 42) (800 gp, 1,000 gp, or 1,200 gp; +10% lbs.). The ability to add the property reinforced to armor itself requires the feat Artisan Craftsman (Dragon #358 39).
- I tried to think of a material that hadn't been used to make D&D 3.5 armor. I'm pretty sure armor made of meat doesn't exist, so one could make that, I guess.
- Unique super-powerful armor is probably the least game-breaking thing one can craft. Crafting one's own unique super-powerful weapons and alchemical items pose far greater problems.
No. Without your gm allowing a house rule, you cannot make armor aside from the categories listed. At best, you can make Full Plate, with a +8 armor bonus, at a DC of 18.
If your DM allows for the epic uses of skills, you can add +10 DC voluntarily to increase how much progress you make in a given day. And if you have time to burn and sufficient access to the material, you could make armor out of adamantine and gain a few points of DR.
Of course, if your GM is willing to house-rule you could plausibly make higher AC armor. But the house-rule that breaks the system the least would be allowing ranks of craft (smith) to count as a caster level three ranks lower for gaining the craft magic arms and armor feat and adding an enchancement bonus to the same, using the magic item creation rules.