Interior designers lack the ability to see the finished product. The builder ususally has a good idea of what it will look like finished in his head. The classic problem is the ID showing up and saying
"I didnt think it was gonna look like that".
You have to hold them to their design drawings and charge for any deviation from that drawing at a T&M rate.
Keep in mind AISD Interior designer certificate is a 16 hr class at your local city college.