For the District of Columbia itself
go here. The linked report includes info on most of the substantial projects in DC proper.
I also posted a thread with major suburban projects many months ago, but as that seems to have been pruned,
go here. That's just for big master plans, TODs and stuff like that. No individual buildings. Unfortunately I don't know of a single place where you can find a lot of info about single buildings in ther 'burbs. There's just too much going on.
This thread, however, gives you info on one prominent skyscraper in Rosslyn.