Expert: Ted Barnhill (Principal, Design45 LLC) - 11/25/2009

Question
I am planning to add an L-shaped addition to my rectangular 21'x25' 1911 house which stands on a limestone foundation.  The two legs of the "L" will include a 25'x8' one story addition to the south side of the house, also a 21'x16' two story addition to the east side of the house, which in total will make a 29'x41' rectangle.  One builder is advising me to build this L-shaped east/south addition on top of sonotubes, while another builder is horrified at this idea and thinks a crawl space is the way to go.  I'm hearing vague reasons about the original house vs. the addition being able to bear weight and settle separately vs. needing to be "bolted together".  My inclination is to go with the sonotube-based foundation, if I know what questions to ask to make sure that it is well designed and will connect well, not settle improperly, etc.  Can you help me some advice, also which questions to ask of my builder, draftsman, and engineer?  Would appreciate any thoughts.  I am in Minnesota.

Answer
Linda,

Your decision of which foundation to use will have several important ramifications for your home, including appearance, energy efficiency, and cost.  For most soil types in Minnesota, either of your proposed solutions (piers or crawlspace) can be used if properly engineered.  Verify that your engineer has considered the soil bearing pressure in your specific location before choosing one system or the other.

Building on piers can be less costly, and make less impact on the site.  Depending on your grade (the slope of your lot), using piers may create problems of how (or whether) to enclose the space below.  Your floors will tend to be colder in winter than if you used an insulated crawlspace, so some homeowners choose in-floor heating when building on piers.  Visually, an addition on piers must be carefully designed to work with your house, whether that involves disguising its "floating" nature or celebrating the difference.  In  most cases, insulating a space on piers to the equivalent of an insulated crawspace will required more (and more costly) insulation.  All that said, there are some who argue that it is healthier to build on pier footings due to the complete separation of the home from (potentially problem-causing) damp soil.

In some cases your insurance rates (or even insurability) may be affected if your home isn't on a "full" foundation, so check with your agent.

Be sure that, whichever method you choose, your builder is able to clearly articulate the strategies to be used in response to the issues above.  When you are hearing "vague reasons" it may be that your builder is relying on hearsay or a misguided standard practice, rather than understanding good building science.  

Good luck with your project,

Ted Barnhill
Design 45, LLC