Cracks in new house slab foundation, how serious

I have rarely found cinder block basements to be dry in the winter. In fact, I expect them to leak. If during the inspection things looked good and dry the report may not have mentioned any hairline cracks because all masonry eventually has cracks.

If the home did not have a sump the problem may be getting worse and was not a problem in the past. I state this because one of the contractors said it wasn't bad enough for reinforcement. For me to try and dry up the basement (digging out, adding drains and back filling with rock) is the best way to go. While it is open a yellow jacket coat or other waterproofing membrane can possibly dry things out, but even doing this I still have seen leaks in block walls below grade.

With all the issues you have mentioned I would expect some mention of the condition to be in the report, but I have also seen homeowners not knowing construction try and take things to court and lose. My best advice is to have professional evaluations done (more than one, and three is better) with quotes and talk with a attorney. The quotes in many cases are free but additional evaluations may cost.

It is possible that if you post photos of the wall showing the bow and cracks folks here might say what I have about block foundations or we might oh that should have been mentioned. Also update your question with the year the home was built.

There are 2 types of concrete, concrete that cracks and concrete that hasn't cracked yet. Houses are essentially floating on dirt and with proper preparation you can minimize the likelihood of a slab cracking. From the picture it does look like a large crack if it happened all of a sudden and is heaving it may cause an issue due being unlevel/trip hazard. But in general cracks in slabs are pretty common. Sidewalks don't crack often because of the expansion joints in between each section of sidewalk but that's impractical for a basement or garage slab. If it's a new build it may be cause for conren or if your house is an area subject to landslides or the sort, you may want to have someone investigate it.