Investing upgrade rooftop deck

[Christopher Bell]

I’m in the process of considering putting a rooftop deck that’s 500 sqft on an unfinished roof on a condo in Chicago IL wicker park. Does anyone have an idea of how much rent per month that would be worth? My thoughts were for a 3 bed 2 BA that would be worth 500 a month more where I could raise rents from 3800 to 4300 which would be 8.5% return on capitol where it will cost 70k to build.