The IRS generally only allows the deduction for the mortgage interest for the portion of the debt that is acquisition debt. So for your first example, only $500k of the $510k would be deductible (≈98%), since the additional $10k is just fees. You can read more about how the IRS treats interest for tax purposes here. For the second example, typically you would deduct the interest on the $520k of acquisition debt and enhancements (≈95%).