Say I buy a house with an attached ADU and finance it with 1 million loan and say ADU takes 25% of sqfootage and primary residence ( "main home" ) takes 75% of sqfootage.
Then 25% of the mortgage interest paid is deducted on SCH E. Period. End of story. Nothing further to discuss.
The remaining is a SCH A itemized deduction subject to the limits as set forth in the TCJA of 2018. If the remaining 75% of the mortgage balance exceeds $750,000, then the SCH A mortgage interest will be limited until such time that remaining 75% gets to $750,000 or lower.
