Most neighborhoods have a price range that they can support. To live in that neighborhood, you'll have to pay at least $X. Once in, you can do all kinds of things to your house, but you cannot do anything to the neighborhood, so a property can be updated/improved/expanded/etc. to the nines, but at some price-point, the typical buyer will opt to go to a superior neighborhood even if that means purchasing an inferior (size, quality, condition) home (There are exceptions to this rule, but this is what is typical).
It really is this simple: Past a certain price-point, a buyer would be paying a premium to live in an inferior neighborhood; and the typical buyer doesn't do that. The typical buyer will move up to the superior neighborhood, even if that means they'd be moving into an inferior house.
In cases like this, the house is only worth what the neighborhood's price range can support. That upper-end price-point is likely not defined by the highest sale in the last 6-months. I wouldn't have an issue going back 3-years or so; past that (in my areas) the market was still reeling from the housing bust, and I wouldn't rely on data from within that market dynamic to use today. Like CAN suggests, I'd go to competing markets. But, I'd want to do a fair amount of research (which might include comparing household incomes) to make sure that it was truly competitive. But you still run into the problem of making sure you can define the upper-end of the value range that the neighborhood will support.
When you arrive at your value, I recommend you do a gut-check: "Would the typical buyer really pay that much to live in this neighborhood... even with the super-house? Or would they move up into the better neighborhood over on the other side of the tracks, even if it means moving into an inferior home?"
In fact, when I'm appraising a home that has been over-improved, I state in my report that past a certain price-point, the typical buyer would opt to move to a superior neighborhood rather than pay a premium to live in the subject's neighborhood. I've yet to have a lender not understand that concept.
Good luck!