I thought you were in MA, not ME. The good news is that if it's the PT that shrank, it's done. The bad news is that if the soil is undergoing expansions and contraction cycles...it's unstable. I don't have a vast amount of experience working in clay, but I had a big bucks job in clay years ago. We actually set up something similar to a drip irrigation system around the perimeter of the foundation. The purpose was that the drip irrigation would keep the clay continually moist so it wouldn't undergo movement cycles as the clay dried and rewet. It'd essentially stay permanently wet and not move. Any excess moisture was removed by foundation perimeter and footing drains. Now, you wrote that the crawl space is sealed up... 1) For clarification, I'm assuming that there is a concrete foundation around the perimeter of this addition? Or is it built on piers? 2) If it's a poured foundation, was a rat slab (thin 2" poured slab) or a full slab poured in the crawl space? Or is the bottom of the subfloor open to soil on the ground? 3) With the clay, did your dad have any engineering analysis done with regards to soil movement or stabilization? As to "what now"...was this a Kerdi shower? Or did it have a topical membrane (HydroBan, RedGard, etc)? I remember your shower but forget the specifics of how it was built. If Kerdi, the cracks are sort of inconsequential when it comes to water penetrating into the walls. The cracks are more cosmetic unless they really open up. I'd get in touch with your installer regarding the details of the walls. Depending on how the walls were detailed/waterproofed might give you a warm fuzzy regarding letting things sit as they are while you see if movement has stabilized versus taking action (caulk the crack, etc) if water penetration may be a concern. I'd mark the ends of the cracks with blue tape and keep an eye to see if the cracks extend or if the movement has stopped or stabilized.

Considering how minor the issue is,I would wait until A. There were more cracks B. The existing crack gets worse C. There was other indication like binding door(s) or drywall crack. D. There was another reason to contact builder. E. Until end of warrant was drawing near then report it if nothing else had happened. As far as you consurn over foundation problem,depends on where you live. If you are in an area that has a lot of foundation problems you should ask local agencys for printed material on how to care for your foundation with grade and moisture. BTW,relax often and injoy the new home.

@millworkman, you mean where shower glass meets shower floor there should be silicone caulking? I believe the GC mounted the glass directly (deeply?) into the grout. I remember asking another bathroom contractor about this and he said it's Old School but works. No? Here are more photos, two of master shower and two of my son's shower. Note the crack along the grout of master shower grout. GC said he'll come and fix once we let him (or anyone) into our home isolation. Thanks for chiming in! I was going to ask about this next. Son's shower Master shower (with grout crack, which we keep dry during showers):

@YG - just get your measurements in order to run numbers through the deflecto calculator over at JBF. Just add your numbers to the post that you already have started over there re: your cracked tile. They like to keep everything in one continuous thread so that members can easily find all the information related to a particular project. Remember, your contractor was required to install your tile in accordance with the manufacturer’s specifications/industry standards. If you didn’t have natural stone tile installed previously, it’s HIGHLY UNLIKELY that your subfloor met these requirements. Plus, your joists may not comply with the right specifications in order for the floor to fully support your marble tile. @Mike Blasko - I am very confused as to why a bucket of ”mastic” can be labeled “mortar” - seems likely to cause a great deal of confusion for a customer. Even if the customer has done some research in order to know that mastic shouldn’t be used, there’s a pretty high probability that he/she would think using a bucket of the ”Ultimate Premixed Mortar” was perfectly fine to use - because, after all, it says MORTAR - not MASTIC. EDIT - I just realized that this wasn’t installed DIY - scary to think that a ”professional” wouldn’t have known that TEC’s product wasn’t the correct type of ”mortar.”