I’ve never used 8821. It’s 4506 and 4506-T that get requested. They should be filled out at signing, especially lines 6, 7 and 9, and limited to the 1040 returns.
When these are dated, the 3rd party only has 60 days to request the tax returns. That is clearly in the instructions. I don’t want my clients signing a blank form.
On a stated income or no doc loan, they cannot check income.
If self employed, they want to confirm that a schedule C was filed.
