Added Value Of Rental Unit To Potential Deal

[Morgan McRae]

I'm looking at picking up a property in Kansas City, MO for fix & flip. It is not quite a deal at current asking price. There's barely not enough spread for profit. The front house is a 2-1, 950 SF. However, there is an additional standalone 1-1, 450 SF house in the back yard which has been rented successfully within the past couple of years. If this were fixed up to rental condition, it could be pitched as an income generator for retail buyers. 

My question is, how do you go about evaluating the numbers regarding my offer price? How much more does this 1-1 rental add to the value of the entire property, along with the main house, a 2-1? I'm sure there are many ways to consider this, but would love to hear anyone's initial ideas. I think this could be a great deal if the back house adds enough value. Thanks.