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If the plans were approved by the Local Building Jurisdiction and then inspected by them to meet Code, there is not liability for the Building Department. They would only be liable if an employee approved something with the knowledge it was incorrect.
The Builder is generally the one the blame will fall on. They had complete control of the building materials, sub-contractors and the site.
If there were unknown soil conditions at the site and everyone assumed the soil was good with no investigation, then finger pointing will most likely start between everyone involved.
Did the contractor have to excavate to build this addition?
Did the contractor have to place fill to build up the site to build the addition?
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