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Those are some pretty nasty looking cracks. The first thing that comes to mind, since you said this was a new addition, is that the new foundation is settling which is causing the cracks. The best thing to do for now is to use a paintable latex caulk and fill the cracks. Touch up the paint when the caulk is dry. Cracks may return as the foundation settles more. Keep dated pictures if they keep reappearing in case you need them later on.
Yes, typically as a new structure settles its foundation cracks will appear. It might take months or even years for a building to fully settle depending on how it's built and the local earth conditions.
If you are truly worried about it, have it looked at by a different professional. While the
Wow, that really stinks. I'm sorry for your bad situation there. You might think about speaking with a lawyer if you think that is appropriate. From the sound of things, it is going to cost you at the very least a few thousand dollars to get things fixed. For the record, I am not a lawyer and this is not meant to be legal advice. Still, I'd look into it if it were me. :)
An older builder should be able to see whats going on. Give him ALL the details, history and documentation of the project.