Mayor's Order 2020-080
Should there be any doubt.
On July 22, 2020, Mayor Muriel Bowser's orders made it clear that a face mask must be worn in the common areas of apartments, condominiums, and cooperatives, along with other facts related to the COVID-19 problems that DC Residents need to know.
Building management needs to control rental prospects, move-ins, advise new residents, and monitor contractors who are not wearing a mask. Once outside, that might be another story, depending where you are or where you might be going. Before entering the building upon your return, you need to apply your face mask.
Check this link https://coronavirus.dc.gov/maskorder for the complete document from the Office of the Mayor.