This looks very vacant.
It's the lack of windows that's the tip off. This building is across the street from the newly opened Marriott Marquis hotel.
The DC Tax office does not have this listed as a vacant property on the assessment database, but it is paying some pretty high taxes regardless. In 2013 and 2012, $29,182.62 was paid each year in property taxes at the class 1 residential rate for a property assessed at $4,156,280. The owners are listed as the Square 369 Hotel Associates LLC of Bethesda, MD.
A quick Google search of the the Sq. 369 Hotel Associates show that they also own other nearby properties. The semi-out of date Blockshopper shows the LLC owning 911 and 913 L St NW. Taking another look at the DC Tax Assessment page, searching on Square 0369 and L for the street, reveals connections to 915 and 929 L St NW, lots with no address number, 0369 0848 and 0369 0805, as well as 1100-1114 Shepherds Alley NW. None are listed as vacant, only one has a Class 3 exception. Several of the properties were bought in 2003. In 2007 Rob Goodspeed documented some of the properties' vacant status in a 9th Street Vacant properties survey.
There haven't been any recent (in the last year), permits for 919 L St NW. Other properties have had some permits for work relating to holding a construction trailer and other hotel construction whatnot have been issued.
Though the DC Tax database has some misspellings and leaves off the 'NW' on occasions, it doesn't seem to be too much of a leap to see that Marriott has some connection with the properties across the street from its shiny new hotel. The listed owner for 929 L St NW is "SQUARE 369 HOTEL ASSOCIATES LLC MERRIOTT INTERNATIONAL INC". So one could gather that at some point the owner will deal with the other side of the road to make its completed property more attractive.
UPDATE 10/21/2014- Short Articles About Long Meetings mentions that 911 L is historically protected. The owner want to raze the building.