Market data is the best way to go. You shouldn't be looking for an amount or percentage that you can use in every instance. The next house will be different. However, if you don't have good market data from current sales there are a few other things that might help you. One of them is the cost approach. Base on the economic theory of substitution, no one will pay more for the basement space than it costs to build. If it's a fairly new home, or if you have good market data on depreciation, then this approach can give you a starting point for your adjustment. Of course, if you have good market data, you should go by that, but basements are tricky. It is not too unusual to adjust one comp at a higher rate per square foot than the others if the finish is better quality. Past experience can be a real help here. If you don't know for sure, you shold consider getting some help from one of your friendly competitors.