Geeze, here we go again. Ok, you are not an appraiser...you are a buyer. You have two properties pretty much the same with the exception one has a brand new $15,000 septic system and the other has an older system that is functional.
If all else was about the same, which one would you buy first? Would you be willing to pay the same for both properties? Hmmm, probably not you say? OK, how much would you discount the one with the old system?
I'm willing to bet your market will say something like $5,000? Now, here is the rub. You are not a buyer...you are an appraiser and you need to make an adjustment for the system that will pass muster with your client. Depending on the price of the property a $5,000 adjustment for condition certainly wouldn't be out of line.
I would just do it, make a comment the condition is superior due to a new septic system installed on XXXXXX date at a cost of $15,000 and be done with it. Cost does not equate to a dollar for dollar adjustment but it certainly is a reasonable basis for some adjustment.