Prorating deductions for short-term rental ADU / carriage house

[Cyrus Murphy]

This is a tax question for the situation where the main house is 100% occupied by the owner and the carriage house/ADU is 100% dedicated for Airbnb renting (never used personally). The ADU is a separate building from the main house, but is the same address (unlike a duplex).

When you calculate your proration for expenses, do you consider un-rented days as personal use days? All the guidelines seem unclear to me.

For example, you rent the ADU 200 days a year and are living in the main house 365 days a year. The ADU is 25% of the property using the sq ft method. I see it as two different ways of calculating this:

1. considered personal use days when not booked method:

200 rental use days / (200 rental use days + 165 personal use days) = 55% and multiply that by 25% (sq ft) = 14% proration

OR

2. no personal days for dedicated unit method:

200 rental use days / (200 rental use days + 0 personal use days) = 100% multiplied by 25% = 25% proration

Since the ADU is solely for renting and is never used personally I would think method 2 makes sense?