Adding a 2 Car Garage: 2 Flat Value Effect

[Felix Z.]

Hello,

I was recently viewing a 2 unit property that lacked a garage. There is a concrete slab out back. How much value-add should I expect to get if I bought the building and added a garage. My hope is that there is some upside due to the climate here (vs. adding a garage on a property, say, in the southwest). 

Thanks!