Hi Dan,I just want to make a distinction, in the RF-1 zone you would be looking at a conversion to a 2-dwelling flat, not an accessory dwelling unit. It's just a nuance in the zoning.
With that said, a 1-hour fire rated assembly is required between the 2 dwelling units (there is also a minimum STC rating required for sound separation). A conversion to a 2-dwelling flat also triggers the need for sprinklers, which in turn requires a separate fire supply line in the front yard (which allows DC Water to shut off your domestic service for non-payment, without disabling your sprinklers). Please note that the 2-dwelling conversion is triggered when you add a second kitchen. In theory a wet bar does not trigger this, but in the last couple years DCRA has rejected wet bars in cellars, because some people convert these to kitchens without providing the fire rated separation. So, if your intention is to install a kitchen, I urge you to show a kitchen and do the necessary work to comply with building codes.
Lastly, the minimum ceiling is 7' for a new dwelling unit in the cellar. There are allowances for 4" drops, down to 6'-8", for structural beams.
Regards,
Steve Fotiu, AIA NCARB
Fotiu Architects PLLC
St...@FotiuArchitects.com