Expert: Richard Taylor, AIA - 3/8/2005

Question
We are contemplating adding a sizeable addition (~35' x 16') to the back of our house.  The current basement is completely underground on the back side.  We were wondering approximate cost difference to expect in crawl space versus full basemen?  Also, how would you tie the added basement into the current basement without doing permanent damage to the existing structure?

Answer
Good morning, Carl - thanks for your question!

There's more than one way to look at the "cost" of a basement vs. a crawl space for your addition project.  In terms of dollars, you should expect the basement to add about 10%-15% to the cost of the project.  That's assuming, of course, that you don't have any unusual underground conditions in your area (high water table, bad soil, rock, etc.).

But there's really no better investment you can make than adding the basement.  It's the cheapest space you can possibly build!  And you can't add it later - once a crawl space, always a crawl space.  So consider the "cost" of the basement in terms of value and use - that basement will add another 500 SF of very inexpensive space to the house.

How you make the tie-in depends upon the soil and the type and condition of the existing foundation.  Assuming the new and existing basements are at the same depth, you'd simply excavate the new foundation next to the existing and continue normally.  If you're in loose soils, then you might have to undercut the existing foundation a little and pour concrete below.  It would also make sense to "pin" the new footers to the existing with rebar.

If the existing foundation is very old and/or in bad shape, you may have to stabilize the existing wall while you excavate.  But unless there's an unusual condition present, you shouldn't have to do anything special.

Of course, you should have all of the structure (especially the foundation) reviewed by a structural engineer or architect, and use the services of an experienced contractor.  This isn't a DIY project!

On another subject, 35' x 16' is a very large addition.  In my experience, most homeowners who add very large additions end up "moving into" them and "abandoning" parts of the existing house.  We'd done many remodeling projects where the first thing we do is to remove a previous very large addition.

We see much more long-term success with a careful evaluation of the existing house, then only adding what's necessary.  The idea is to make the entire house work; the solution to many remodeling projects is not adding more space it's reworking what you have to satisfy your needs.  As a bonus, such an approach costs a lot less!

Assuming you can reduce the size of your addition by reworking some of the existing house, you might save enough money to build the basement!

Hope this helps!

Richard Taylor, AIA
Richard Taylor Architects, LLC
www.rtastudio.com