Hi Leo,
It looks like others have already provided good feedback, but I just wanted to add some thoughts, especially regarding parking.
Per Zoning Subtitle C Section 701.5, a residential flat development must provide one parking space per two dwelling units. The existing single dwelling use is required to have 1 parking space, but since no parking is currently provided, a parking credit is granted for one parking space. Since only one parking space is needed for 2 dwelling units, and a parking credit can be granted, no new parking is required. This is alluded to in Sections 705.1 and 705.2, but it's not clearly stated. As a result, I have encountered DCRA zoning reviewers that are unaware of this parking credit, but if you push back, the Zoning Administrator eventually overrules them.
As for sprinklers, I'm aware of the previous post that stated they are not required when square footage is not added, but I have never attempted this argument with DCRA. In my experiences in the RF-1 zone, a change of use, regardless of increased square footage, triggers the need for sprinklers; but only in the portion of the building that is changing use, for example, a cellar being converted to a separate flat, not the upper levels that were already a dwelling unit. If you do indeed need to provide sprinklers, then DC Water will require you to install a separate fire line serving the sprinklers; which allows DC Water to shut off service for non-payment, without disabling the sprinklers. DC Water may waive this requirement if the property currently has no curb stop (utility company shutoff) on the exterior, and the only shutoff valve is inside the property; but if the flat conversion requires upgrading the water line and/or water meter, then DC Water will also require a curb stop be added on the exterior, and consequently a separate fire line.
I hope this helps!