I didn't say it had to be adjusted / revised each year.
It should only be adjusted when you make a significant principal payment towards the debt. Other than that, it will remain constant.
Let's use a "worst" case scenario and you paid $750,000 in principal. At this point in time, the personal portion of the debt has all been paid off and all interest expense is attributable to the ADU; it is no longer 75/25.
So the same analogy when you paid the $30,000, while this is certainly not significant, over time this could make a difference depending on principal payments made.
So in this case, you paid $30,000 and the split is $720,000 / $250,000 = $970,000 = 74/26 at this point.
If you paid down another $100,000, the split is now $620,000 / $250,000 = $870,000 = 71/29 percentage.
*A reminder that posts in a forum such as this do not constitute tax advice.
Also keep in mind the date of replies, as tax law changes.
