To be safe when picking comps, you want to identify several houses that are a very similar product to what you will be building and average them.
You can use a real estate agent to assist in making sure the comparative sales you have chosen are in the same areas an appraiser would consider for choosing comparatives sales from.
The more you adjust the value the more subjective it gets. I have seen people add huge amounts of value because their property had more square feet than the comps.
By increasing the search radius for comps, there is a greater likelyhood that the comp is in a location that is either in higher demand or less demand than the property you are looking at.