Michael Hiller and his small firm have made headlines in New York City for fighting to preserve landmark buildings and structures under threat of being erased forever by the wave of new development that has crashed over the city.

But Hiller’s firm, Hiller P.C., which brands itself as a protector of the “little guy” in the face of well-heeled developers and government entities, is also making a name in an area of law that may seem far afield from land use—by taking on cases on behalf of cannabis users.