We’re in the process of planning an addition to the back of the house, some form of screened-in porch. We live outside of Chicago and the city requires any additions to have a full foundation – we can’t just put the porch on top of the existing deck with pylons.
How much more would it cost to just go down all the way to 8 ft and make it an addition to the basement too? I’m not looking for exact numbers, I’m just curious if this is something that is feasible. We already have a basement, but we suffer from TMSNES (Too Much S#!+, Not Enough Space). Another 12×12 foot addition to the basement would be awesome.
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Can't help you with exact costs but wanted to recommend the full basement.
In years gone buy it was a no brainer to add the rows of block necessary to give one extra headroom beyond the 4ft frost depth.
Unfortunately, many still stopped short by not adding the last block row or two and made do with a 7ft high basement
which after later adding mechanicals turned what could have been a basement into what is essentially only a stand up crawl space.
Regardless if a block or poured wall, the extra costs will be minimal in comparison to what you'll be receiving in the process.
Often those extra costs aren't that awful much more than the price of materials and simple labor as much of the overhead of setup and equipment is already necessitated by the fact of the four foot depth.
So the extra excavation charges and the poured slab costs might be the main points to address.
I would go with a full basemant. Pricing for something like this is best done by those who live and work in your area. Way to much variation across the country to get good figures any other way.
Might depend on how 'screened-in' your screened-in porch is.
Chicago is on the edge of the 54" frost depth. It may not be so much to dig a little further for a full basement. But your porch might need to be treated more like an actual addition so errant water doesn't get through the walls/floor. Which also affects your insulation and finishes.