This varies per payor. I hate to be vague, but each payor is different. It's best to talk with the insurance company and see what they accept. For some, we've found that it's best to send the preauth with the claim for the buildup. This way they’ll approve the crown and pay for the build up without any question. Just in case they do not want to pay for the crown without the buildup.
But something to keep in mind... always bill for exactly what you do. My first instinct is to say that the buildup should be billed on the date it's completed, but again, check with the payor.