The ghosts of Washington's law firm offices may take years to vanish.

That's because office space in the city is at its highest vacancy rate in 25 years, a chronic oversupply that has left massive office buildings—including many formerly occupied by large law firms—dark and deserted.

As of last month, 6.5 million square feet of new or renovated office space sits open in D.C.'s downtown and in the city center's East End, the two neighborhoods firms most often haunt, according to consulting reports from the real estate brokerage firm CBRE Inc. Much of that space was once home to law firm tenants.