What you have to do is apply for a "Variance". Every town is different but ask how you can do that. No building inspector will EVER volunteer that you can go that route, but the clerk at town hall or the building office may be able to help you.

We can't run pools lengthwise in our town, only cross-wise, but the people behind me are doing it--they got the variance. (they just can't get the contractor to plaster the darn thing!)

With an AG pool it shouldn't be a problem. The reason you WANT to get it approved is that in the event something bad happens, your home-ownders insurance company can stiff you and refuse to pay saying "it wasn't an approved structure".