"and sell it with their home, that there was a way to expense it"

You really have a mix of things in your consideration.

A Building of significant cost would be Depreciated, and might qualify for Bonus Depreciation. It isn't Expense, directly, because it is standing right there = Investment. I teach this as the difference between Expenditure and Expense. We spent money, but there is something tangible to show for it.

You describe it is 100% used for business, but part of the residential property and likely to be sold with it.

Have you considered that the square foot method might work better? It's called Simplified, for a reason.