Need to change Deed?

[Jorge Esteban Vargas]

I'm about to consult with an attorney but wanted to get input from this community as well. My wife and I own an investment property in Forest Park, GA. We want to turn our SFR into a duplex by building an additional unit. We're close to finding a partner and are willing to make them 1/3 owners of the whole property once the construction project is completed under 2 specific conditions:

1. They would only get half the profit of the new-constructed unit. Right now, the property is ~1,200sqft and the new unit would be about 960 sqft making the overall property ~2,160 sqft. 

2.If we decide to sell the whole property in the future, then they'll get 1/3 of the proceeds.

My question is should we change the deed to make them 1/3 owners of the property and then have an attorney write a detailed agreement or can we simply have the detailed agreement state they're 1/3 owners and will get half of the profit of the new unit?

For tax purposes, will they be able to use depreciation if they're NOT in the deed?