The Ironskin spell states as follows (emphasis mine):
You gain a +4 enhancement bonus to your existing natural armor bonus (if you do not have a natural armor bonus, you are considered to have an effective natural armor bonus of +0). This enhancement bonus increases by 1 for every 4 caster levels above 4th, to a maximum of +7 at 15th level.
I have a Full Plate armour with a +5 enhancement bonus, and no dex bonus. At caster level 20, what would my AC be? Specifically, does the enhancement bonus to natural armour stack with the enhancement bonus of the full plate armour?
- 10 + 9 (plate) +5 (armour enhancement bonus) +7 (natural armour enhancement bonus) = 31 (with stacking)
- 10 + 9 (plate) + 7 (natural armour enhancement bonus) = 26 (without stacking)
This is different from other similar questions (shield spell and shield enhancement bonus and enhancement bonus to armour) in that I need to understand what exactly is an enhancement bonus to natural armour. Being that in the AC tab of the core rulebook it's specified, in the other AC bonuses section (emphasis mine)
Other Modifiers: Many other factors modify your AC. Enhancement Bonuses: Enhancement bonuses apply to your armor to increase the armor bonus it provides. [...] Natural Armor: If your race has a tough hide, scales, or thick skin you receive a bonus to your AC. [Core Rulebook, pag #179]
From this it seems any enhancement that affects AC is lumped in one category, but it's not very clear. The question about enhancement bonus to armour would provide an answer, which is:
You don’t get enhancement bonuses to AC, you get enhancement bonuses to armor bonuses
But it lacks source and thus is non conclusive. If anyone could find a source on that then we'd have an answer.