ADU investment value relative to total property

[Matt H.]

Hey everyone, I am new to the Bigger Pockets forums and have a question regarding a potential transaction. My girlfriend and I live together in a house that she owns. The home has a detached garage in back and is zoned for a possible ADU/carriage house. Our potential plan is to co-finance the construction of an ADU for short term rentals as 50/50 partners and then split the profits interest. This would all then possibly be run through an LLC to manage the short term rentals. The question we are having a tough time with is how to value the investment in the ADU relative to the total property. We are hoping for the best and planning for the worst. So, if something goes awry in the relationship and we break up, we want to make sure that we have an agreement in place which somehow tracks what my equity interest would be in the ADU relative to the total property. Thus, if she wants to buy me out or decide the sell the property we have a fair benchmark with which to evaluate my interest. Any thoughts on this would be greatly appreciated!