How much does a 2 car detatched garage add to the value of your house.?

We have a ranch house with an 2 car attatched garage. We built a 2 car detached garage out back for our restored vehicles. So now we have a total of 4 garages. We are thinking about relocating and wonder how the extra garages will figure into the value of the property. Does that count as square footage or not. Or do we figure in the cost of what it was to built. How do appraisers figure this out. Please appraisers, bankers or realtors only. Thanks