@Drew33820 as others have stated the mortgage has nothing to do with this. Why? because the IRS is looking at the increased value of the ASSET. the LIABILITIES (the mortgage!) has nothing to do with it.
Don't you have documentation of what you paid the builder? even cancelled checks, bank account statements, etc. that support your cost to build?
Moving for a job has no impact on the equation. If would affect the $500,000 exclusion if you had lived in the home less than 24 months. This is often misunderstood: moving has no impact on the gain calculation; only the exclusion calculation.
Real estate has inflated since 2020, but that much??????
