Most people looking for a simple and/or small addition assume they can save money by putting it over a crawl space rather than a basement. I always advise them to at least ask for pricing. They are often surprised at how little it costs in the scheme of things, and in a certain sense it means getting “free” space: “free” in the sense that the added square footage won’t show up in tax assessments but will show up in loan appraisals for refinancing, and in real estate advertisements when it comes time to move.
Footings are required by building codes (and wisdom) to be below the frostline; here in Northern Virginia, that means 30″ below grade. Usually the first floor of a house is 30″ or so above grade, so that’s already a delta of 60″– 5 feet– between the first floor and the bottom of the excavation. The slab which is the floor of the crawl space– and I always recommend at least a thin “rat slab” rather than dirt or plastic membrane– can be poured directly above the footer. Considering that a basement is eight or nine feet below the first floor, that means you’re already digging out a good deal of the dirt 2/3 of the way down to the basement footer.
In addition (a pun that cannot be avoided when writing this kind of thing), the new footer is going to step down to meet the level of the existing basement footer so as not to put undue lateral pressure on the existing basement wall. This means you’re digging down even further at the area closest to the house. If your house is on a sloped lot such that it has a lot of exposed basement wall at the rear, you should almost certainly pay for the extra few cubic yards of dirt removal to get a full basement.
I’ve never had a client say to me, “Gee, I really wish we hadn’t paid for the basement under the addition.” On the other hand, I’ve had more than a few say “I can’t believe we weren’t going to dig this out. The added space has been great, and definitely worth the additional cost.”