You might reach out to real estate agents/brokers active in that area and get their thoughts on the situation.
Note what Pete says above. This is one of the six acceptable ways to derive an adjustment.
You sound confused about trying to get a tax appeal in assessed value and your role as an appraiser to perform a MV purpose appraisal.
How many tax appeal situations have you been involved with? Your posts tell me zero.
I am using the common definition of MV which would an include a discount for the wall repair, but if this is true, then everyone who has a substantial house repair could request an assessment discount which is illogical.
First of all make sure you are using the correct definition for this type of assignment. The MV definition for lending or for the IRS or for certain states are different and especially in tax appeal cases.
Most definitions of market value include words about the buyer and seller being well-informed or acting in their own best interest. Find out the correct definition used in PA.
We all learn in appraisal 101 that cost does not equal value and it is also the case that the cost to cure does not equal the discount.
If a house needs a new roof and that cost is ~$25,000 then a typical buyer MIGHT say, OK, I will buy this house for $25,000 less than an almost identical home or they may say, I will pay $30,000 less than the house with a good roof because of the hassle of finding a contractor and dealing with the mess and headaches of the project.
I feel like I am forgetting to factor-in something.
You might be forgetting the "hassle factor" or "EI". I think your situation might be more serious than a roof in that it is a water penetration problem and it could be more of a market detriment than a simple roof. "EI" takes into account the cost, risk, time, talent and management of doing something.
The COST might be $25,000 but what buyer wants to take on the risk that it might be more expensive without being compensated for that risk and ALSO the time to address the problem, the talent to take on the project (not getting screwed by high or low bids, etc.) and the management of taking care of the problem?
Around here the cost is multiplied by 10% for contingency which also has to be factored in PLUS EI which for "typical (your case is most likely not typical) EI is 20% or $20,000, whichever is more.
As a wild guess, your situation MIGHT call for a $50,000 reduction in value, but that is a wild guess based on what you have provided and based on MY market.
Getting back to their request for a reduction in taxes based on a $25,000 reduction in assessed value seems kind of goofy to me. Around here that would only reduce property taxes by at most $400/year; north of me it would be about $800 at the tippy top. I assume your fee is a whole lot bigger than that.