I'm glad you were able to work it out and get a credit. I hope you will still be able to take advantage of the CWC program. Note, the program is from NYC Department of Environmental Protection for any areas that drain to the reservoirs. The NYC drinking water that comes from the reservoirs west of the Hudson isn't filtered, so these programs are important to make sure the water quality says high enough that it doesn't need to be. I interned for NYC DEP in Kingston though it was in a different department so I'm not sure exactly how this program works. There is also a whole team on conversation officers that are working to stop (and fine) pollution in the watershed, though I don't know if they deal with septics or not.
Do you mind me asking how much you are budgeting for the septic? I saw the CWC said $15k was the average, but I've heard horror stories of $50k septics. If you have gotten an estimate yet, the contractor or engineer should have done a perc test to get an idea of how the soil drains. If more material will need to be brought in that will increase costs.
I'm very curious as to how this deal will work out for you. I would love to break into the STR market in the Catskills, but I don't think there are many great properties right now which offer multiple exit strategies. With the market for selling up 30% I'm assuming resales will be at a loss. I haven't ran the numbers on long term rentals in the Catskill park but I know the average incomes are low. It's different when it's a blend of personal and investment as it sounds like this is for you. My first property was as well, and the long term rental profit isn't great.. hence why I'm looking at short term. Lexington in Greene County just had a board meeting and she mentioned something about Ulster County as a whole, though I'm not sure exactly what they were thinking.