That's a difficult one.
If you merely rented out the garage - it would have been part of your personal residence for the last 2-out-of-five years, so it could all be covered under that.
If you had built a brand-new building - it would be completely separate and you would have paid tax on the gain due to that portion.
But you took a portion of your former residence/usage and remodeled it into a new "dwelling unit". Looking at Regulation §1.121-1(e) and other things, I don't see any clear-cut answers.
To me, it would seem logical to assign the original garage/building as part of the original residence, and 'exclude' the gain from that. But then all improvement cost consider as a separate property. For example, if the original garage was $20,000 and you added $40,000 to convert it into a new ADU, I would think allocating 1/3 of that to the principal residence and 2/3 to a separate, taxable unit would be most logical. But as I said, I'm not aware of anything clear-cut in that regard.
As side notes: the 1.5 years over 4.5 years has no effect. It is probably all-or-nothing, and is based on if it meets the 2 year rule (in your case, if the ADU was considered part of your personal residence/usage or not). Also, I wouldn't assume your 24% based merely on square footage. A separate building will usually have a different valuation, not merely based on total square footage. I would get the house versus ADU appraised.
