If a VA appraisal comes back with a significant error, you can challenge the report. In some cases, you can present facts about the property that the original appraisal may have overlooked. For example, the appraiser may have missed the fact that you are on city water or have miscalculated the square footage of your home.
With this document submitted, the appraiser may reevaluate their determination and adjust the property's value. If not, you can appeal to the Department of Veterans if you are trying to adjust the property's value by more than 10%. At that point, a VA employee will look at the documents to reissue a valuation.
If you are pursuing a VA Streamline Refinance, an appraisal isn’t required by the VA. However, your lender will likely require an appraisal to finalize the loan.