R2 Zone options - ADU versus Boarding house

[Dave Park]

So my client bought a three story home in an R2 zone in NE DC, planning to convert the first floor into a rental unit.  Promptly discovered that R2 is a single detached house zone and tried to back out of the deal.  Researched uncovered that Accessory Dwelling Unit (ADUs) are allowed in R2.. yet seems to require a $4000 statement of occupancy.  My closing attorney brought up the option of using it as a "boarding house." does that require a statement of occupancy?  Don't see much via google.  any advice on this would be appreciated!