Brad has provided you with very accurate data. Basically the livable space is that which you base the price on. If the walk-out is unfinished, I would place very little value on it, if it is finished, and has interior access, then the value would work into the main level living. We have found that if you take 2-3 appraisers, each will treat the valuing of this differently. Then, we would have to ask what would your use be? Would the walk-out be a portion of the home that you woud have an infrequent use, or very much use? I would base the suggested offering price on the main living area.

Interesting but good question. If you are buying you can see the differences in the market as you shop. The market changes on this as well as anything else. This is a question an appraiser should answer. When I help pricing a home for sale I would likely compare 2 level homes with 2 levels sold homes. That being said you may not have this option. If you use the cost approached you would take the value of the lot and the cost of the home and subtract the depreciation.
But for a more simple answer it depends on what is on what level, what is the finish. But, usually 2/3 - 1/3 that of a single level, depending on the usability.
If you are purchasing and you find a home you like that have 2 levels. And nothing else compares in a single level at a similar price. Then the 2 level is priced close to market.
Happy Hunting,
Brad

It depends on the quality of the finishes. It can go from equal to the upstairs to 1/2 of the upstairs price/sq ft. For example, if the walkout is complete with bathroom, drywall, flooring etc, then the price per square foot could very well be the same as the upstairs price per square foot. However if it is only a finished area for a pool table it could be as well as 1/2 the value as upstairs. In our area it would be a case by case basis based on the quality of the space.

It really depends on the market area- try looking at solds single level vs 2 story in similar condition with similar sq ft and see if a pattern emerges. Also think of who is the buyer- if the neighborhood is more family oriented, the adjustment may be very small or non-existent. If the neighborhood/area is frequented by retirees, your adjustments may be much more substantial, especially if time on the market is a factor. Also, how many homes are competing against it- that always comes into play as well. Sometimes pricing a property is more of an art than a science.... Hope that helps.