An averaging technique is not acceptable per Fannie Mae Guidelines. See below...
Joe Moore
Fannie Mae Selling Guide
Section 409 – Valuation Analysis and Final Reconciliation
The valuation sections of our appraisal report forms enable an appraiser to develop and report in concise format an adequately supported opinion of market value-based on the cost, sales comparison, and income approaches to value (as applicable), and, in the case of small residential income properties, on comparable rental data. If the appraiser believes that additional information needs to be provided because of the uniqueness of the property or some other condition, he or she should provide additional supporting data in an addendum to the appraisal report form.
The reconciliation process that leads to the appraiser's opinion of market value is an on-going process throughout the appraiser's analysis. In the final reconciliation, the appraiser must reconcile the reasonableness and reliability of each applicable approach to value and the reasonableness and. validity of the indicated values and the available data, and then must select and report the approach or approaches that were given the most weight. The final reconciliation must never be an averaging technique.
If the appraiser has provided a comprehensive and logical analysis of the neighborhood and the property, the lender's underwriter should be able to reach a sound conclusion on the adequacy of the property as security for the mortgage.