Answers

I would presume that there are restrictions in Washington which require the approval of an Urban Design Commission for any archietectural modification to an older structure. Such organizations operate with guidelines that are unique to your area.

I would suggest that you contact your local Building Department. Provide them with the name of your neighborhood and inquire of them  "what if any restrictions limit your options".

You will need to contact the DC Zoning Department to get information regarding your property setbacks. Once you get the information on restrictions or limitations in your area there can be a few ideas such as a closed in porch or even sunroom. I have a relative who lives on S Street and he built a two level deck on his townhouse. The areas, if similar to his, are somewhat small and narrow but can be manageable for new construction. Good luck on your project!