This is an appraiser's nightmare. Typically speaking, you want to find homes within a 5 mile radius that are similar--that have sold within the past year. Sometimes it is necessary to expand the area and/or go back farther than 1 year. With the way the Real Estate market has changed during the last year, I would first widen the area of my search and hope to find at least 3 reasonable comparable sales (comps). Since you don't have to do an appraisal, you can look also at active listings that are comparable. This tells you what other sellers in the same market area are thinking. The age of the homes that you compare your subject home to need to be in the same age bracket--I would say 10-25 years old. A good Realtor in your area should be able to help with this process. Good Luck!

A realtor should be able to comp your property unless it's extremely unique. 18 years is not that long ago when speaking of construction. You can also hire an appraiser. Good luck!