Interesting one I found out from my broker...
Banks dock you with a nominal rent expense of $700 per mth even if you still live at home from your folks.
However if you are from certain ethnic background where living at home well past your 20s is the norm (Asian, Greek, Italian)... you could prove that you still live at home and get it waivered!!
I assume policies vary from lender to lender but an interesting one to explore nevertheless
Haha i'm pretty sure ethnicity isn't in lender policy. For some lenders, as Jamie said, a letter would do. Others have it as a necessary part of their servicing calculator.