How Do I Estimate The Value Of A Second Structure On My Property?

zillow.com and other web sites will give you the estimate of your property value. However, if you have built a secondary structure on you property, how do you account for this value. Do you simply add the square footage of the structure to the main house and calculate the new value based on the average $/square ft? I think it should be more since it is a separate structure. My structure, for instance has electricity, plumbing, a bathroom, heat, air conditioning etc… and is 14′ X 34′ with french doors two windows, and a fireplace.