We are in NJ. The foundation is a precast foundation. It is poured offsite to spec and the framing \ insulation is all built in. It's pretty cool actually. They just truck it onto your property, crane it into place and boom, you have an addition. See photo of them swinging one section into place
They arrived at my property at 7:30 AM and pulled away at 11 AM. All set, we could put the deck on it that day (but we waited until the following day). There were 6 sections, and they are attached with bolts and glue. I was told that the glue is so strong that one time when coming back to try to correct something, they tried to separate the panels and ended up breaking the foundation while the glue held.
The foundation needs no water proofing as the conditions in the plant are so good that they density of the foundation is far superior to that which could be achieved on site.
It's more expensive, but the timing of our project was such that it made sense. A traditional foundation would have delayed us a few months, and this we could pop right in and work through the winter months. I'm very pleased with it so far.
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