Do bracers of armor stack with magic armor enhancements and special abilities?

Question

I am aware that bracers of armor's armor bonus does not stack with the basics armour bonus, but I wonder:

1- does the armor effect of bracers of armor stack with the enhancement effect of an armor? (following the example below, would the PG have a total armour bonus of +9?)

2- Can I use the special abilities of the armor if I also use braces of armour?

For example: If I wear braces of armor +8 and I also wear a leather armor with enchantment bonus +1 and special abilities (e.g. fortification heavy and invulnerability).

Answer 1

Bracers of armor do not stack with magical armor enhancement bonus, but you can benefit from special abilities

The stacking rules about Enhancement bonuses state:

An enhancement bonus represents an increase in the sturdiness and/or effectiveness of armor or natural armor, (...) Since enhancement bonuses to armor or natural armor effectively increase the armor or natural armor's bonus to AC, they don't apply against touch attacks.

So the Enhancment bonus is improving the armor by increasing the armor bonus.¹ The general rule for armor bonuses states:

The armor bonus from a suit of armor doesn’t stack with other effects or items that grant an armor bonus.

Since the Enhancement bonus increases the armor bonus, it does not stack with other effects that also grant an armor bonus, such as Bracers of Armor.

The rules only state that the armor bonus does not stack with other effects that grant an armor bonus. You still could benefit from other special properties of your magic armor. As the stacking rules state that "If the modifiers to a particular roll do not stack, only the best bonus and worst penalty applies.", in this case you would benefit from the higher armor the bracers provide, and the other special properties the armor provides.

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^{1 It is fundamental for being able to use a magic armor and magic shield together that Enhancement bonus counts as an armor bonus (or as a shield bonus, if on a shield). If it did count as an Enhancement bonuses you could not benefit from the magical improvement of both, as bonuses of the same type would not stack.}

Answer 2

1- does the armor effect of braces of armor stack with the enhancment effect of an armour? (following the example below, would the PG have a total armour bonus of +9?)

No. Magic armor and bracers of armor each provide armor bonuses to AC, and typed bonuses do not stack with other bonuses of the same type. So a +1 chain shirt’s +5 armor bonus to AC would not stack with a bracers of armor +3’s +3 armor bonus to AC: you would only add the +5 armor bonus to your AC if you wore both of these items. (There is a slight benefit in that incorporeal touch attacks would ignore the +5 but could not ignore the +3, so wearing both would allow you to get the +5 against most foes but also get +3 against incorporeal attacks instead of +0. This is not generally considered worthwhile; if your campaign has that many incorporeal enemies, get ghost touch, it’s better and cheaper.)

Note that a +1 chain shirt gives a +5 armor bonus to AC. It does not give a +4 armor bonus to AC plus a +1 enhancement bonus to AC. Rather, it has a +1 enhancement bonus—to its own armor bonus that it applies to the wearer’s AC.

2- (following the example below) can I use the special abilities of the armour if I also use braces of armour?

Yes, you can, but this is very unlikely to be cost effective, since your bracers of armor need to overcome the basic armor bonus of the armor plus the mandatory +1 enhancement before it can provide any benefit, and that is immensely expensive.

A better approach, if allowed, is to put the armor properties on the bracers of armor. The 3.0e supplement Arms & Equipment Guide allowed this, but it is unclear if those rules are legit in 3.5e (strictly speaking, there may even be questions about them in 3.0).

An even better approach is to just ignore armored AC. It’s the worst defense in the game; take your +1 chain shirt and then just stop caring about AC. You want special armor properties. You might still want to use the bracers of armor for even more special properties, but not for AC.