Cost of building a garage from scratch + options

[Adam Craig]

We are about to start a flip with and the house has no garage. The city requires one and we bid accoirdingly. I am in the process of getting quotes from "garage builders". 

I know I might be paying a bit of a premium for the guy that takes is from start to finish - permits/pad/structure/roof/siding/electrical. Some quotes I received so far were 11K-15K for basic 20 x 20 2 car garage but I have more coming in.

Ive been told I can get it much cheaper if I call a lumber/building supply store and get a "garage kit" and have someone build it. Any thoughts on this? I was also thinking of getting the concrete pad done because I have a good priced concrete guy then getting quotes from there?

I like the idea of the guy that handles all of it but at what cost?