There is no "set rule" for basements. Variables to consider include finished/unfinished, quality, condition, layout and overall utility. Almost everyone would want a basement if all things were equal but there are buyers that can be unwilling to pay extra for that. Remember, under NO circumstances are finished basements considered in the overall living area of the home.
From an appraisal stand point we'd want to try and isolate the contributory value of the basement by isolating that feature - much as you described. There are always other variables to consider with each home (location in the community, age, condition, quality, view, seller motivations, market at sale, conditions of sale....) that are influences; this is an imprecise arena.
I'd say provided the basement is the only significant difference, 5%-8% is reasonable for unfinished areas. Email me the communities and I can render a better opinion.
Hank Miller, SRA, ABR
Associate Broker & Certified Appraiser
RE/MAX Greater Atlanta
A few years ago I did a detailed analysis to determine value and sales times in Echo Mill sub in Powder Springs. http://www.EchoMill.info On average a basement added about $30,000. If it was finished it added a little over another $30,000. The homes with basement finished or unfinished sold in about half the time of the homes on slabs.
You can see all homes for sale in Powder Springs at http://www.PowderSpringsHomes.info
If you would like market reports for a particular subdivision are school district please let me know.
I see that you have received varied responses. I think that $25K-$30k is reasonable to add for a finished basement with similar square footage and in the same neighborhood. Now in the event that the seller has finished a $100K basement than most likely the seller will not get this back! The most important factor here is, What has sold in the community with a finished basement and for what price? The listings don't matter as much as the sold comps with and without basements. Hope this helps! :) Valerie Corpora-Ellis 770-262-8480
Very old -school numbers ranged from $25K to 50K just for the basement differences.
In today's market, basement CAN add value to a home, but it's not a "shoe in" that it is going to cost more or add value in today's buyer's mind.
That being said, it'll be a good future investment.
Does the basement add a 10% factor? Absolutely NOT. The cost for a basement is relatively fixed and the same for square footages.