I had this same question, by the letter of the rulebook it would seem that you would add your level so the AC would be 28 in your example.
I was unsure of this so I turned to the bestiary and found the NPC section toward the end. It was there i found the level 3 NPC Cleric of Rovagug who is wearing chain mail. In the NPC entry this cleric has a dex of +0 but an AC of 18. Seeing as chain mail provides a +4 there is still another +4 missing. It would be reasonable to assume that this cleric is an expert in medium armor but nothing higher as it is only a level 3 NPC which accounts for a +1 to AC. This leaves us at 15 AC. So it would dictate that we must also be adding this clerics 3 levels to AC giving us the 18 AC per the NPC Stat block.
In conclusion yes it would seem you add your level