First of all, this appraisal is not for a sale. It is an as-is appraisal for the lender that has this property in their portfolio, and they want to know how much they can get for it
If they want to know the as is value, then the cost of repairs is a meaningless metric. Secondly, that bank has no intention of hanging onto this forever. Who is to say they won't use your appraisal as a marketing tool to a buyer?
Reconstruction value refers to repairs - Insurance v appraisal. Insurers have to pay for demolition as well as construction. "Repairs" cost more per SF than new construction. The books are different and so is the pricing. Typically the cost is about 10-15% higher for reconstruction cost v replacement cost.
So where is the line? Where is the line between "repairs" and "construction"? I will err on the side of caution, because unlike you, 99% of appraisers are not likely licensed contractors. They have no business doing extensive repair estimates.
a cost estimate is not rocket surgery. I mean, the price of drywall is X, it's a 100 foot section that needs drywall replacing
Ever had a repair bill that was higher than you thought it ought to be?
Did that spot on the rug you thought you could fix turn out to be from a leak in the kitchen on the other side of the wall? And that leak turned out to be from a rusted garbage disposal, which when you took it out, the cut off valve couldn't close completely, and the top ring was frozen into the sink? When you took the sink out to get to it, you find the faucets look bad and notice the countertop has rotted around the faucets...and on and on? Repairs have the annoying habit of metastasizing into a larger project than is obvious.
The savvy flipper might cover these up, use used parts, etc. Most contractors I know bid with a contingency clause inserted in the contract for the unseen. As appraisers, we are not only limited to the visible, but when we step outside our area of expertise, there is a question of whether your E & O is there standing by you...licensed contractor or not. If licensed then you need contractor's E &O. If not, you are dabbling in something best left to professional estimators. Repairs, beyond minor defects, are not our job. And $50,000 can be half the cost of a new house in my neck of the woods.
I don't do new construction without someone else's plans and specs and I don't do repair estimates best left to a contractor. And for the average appraiser to do them seems awfully high risk potential to be uncovered from your E & O and increased chance someone will buck up over the estimate. Never mind the issue of competency and whether or not you used a good cost book, know how to read it, and know enough construction to know what is actually going to be required...(2 coats of paint or 1?, do I use the local multiplier for a dumpster or do I assume they haul off the trash themselves, etc. etc.)