Congratulations! Home buying is a huge step and you took it!
There seems to be a difference of about 150 SqFt. I would recommend you to talk to an appraiser to ensure the accuracy of the sqfootage and if there is a significant difference then what you were told by the builder then you may have a legal case against the builder.
The appraiser can confirm when he does the appraisal, he will measure as standard practice and request to be present when the appraisal.
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