That portion of your primary residence that you are renting out, is the percentage that is "exclusive to the renter". Generally, that's a single bedroom. Will include the bath also, provided the house has two or more full baths, and that bath is only accessible from the bedroom that is being rented, making that bath exclusive to the renter also.
While 30% being exclusive to the renter as a bit high, it's perfectly possible if say for example, you have a 3 bedroom/2 bath house where you are renting out two of the bedrooms, and the one bath is between the two rented bedrooms and shared by only the renters. That makes it exclusive to the renter. So I could see 30% rental use in such a situation.
Your retaining wall is not "exclusive to the renter". So you don't really need to bother entering it. Since it's zero percent business use it has no effect on your tax liability what-so-ever. However, if you want to enter it, since it does increase the cost basis of the property overall, you most certainly can. But it's business use percentage will be zero percent since that retaining wall is not exclusive to the renter. All entering it will do is increase your overall cost basis in the property. It doesn't change the amount of your depreciation though, by a single penny.
Basically, since that wall is not exclusive to the renter, you don't need to bother with it or concern yourself with it on the tax front, until the year you either sell the property, or convert the entire property to 100% rental business use.
