The Brief A Seattle developer converted vacant South Lake Union office space into apartments, but says most office buildings aren't profitable to convert. Downtown Seattle has the nation's highest office vacancy rate, with about 20 million square feet of empty office space. (FOX 13 Seattle)Connell’s dilemma reflects a vacancy crisis across downtown Seattle, where the Downtown Seattle Association estimates roughly 20 million square feet of office space sits empty. Office buildings in downtown Seattle (FOX 13 Seattle)By the numbers:Even when conversion projects succeed, the financial returns can be slim. "You're taxing housing to build housing," Connell arguing that mandatory fees, combined with stringent local energy codes, directly escalate construction costs.