Hello- I agree with below. You are spinning your wheels until you speak with the town's building inspector, and also someone from the Zoning Department, if needed.
I assume you have public sewer service? That makes it a bit easier.

what about parking offstreet for an additional tenant? That could be a problem.
Once you find that stuff out, let me know if you need additional help. I know a few good local RE attorneys that could make it easy, and they are reasonable. Thanks and good luck,
Ken L.

If your property is currently zoned for a 2-family, the process will typically involve applying for a "Variance". In New Jersey, the property owner (with the assistance of an attorney if the property is deeded to a corporation) would preparare a plan and submit it for review. (That is the simplified explanantion).

Contact a local attorney who is experienced in working with your local township in obtaining "use" and "zoning" variances.

Robert Greenblatt
Keller Williams

The best place to start would be with the building inspector. He will be able to tell you if it is possible and if so what the process would involve,