I dropped by a presentation on the health of the Downtown DC Business Improvement District recently to hear an update on the area’s ongoing resurgence.

The business leaders had plenty to say about their successes, of course, patting themselves on the back for snagging this restaurant and celebrating that anniversary. But they also were not shy about discussing their challenges, displaying a kind of candor I don’t often encounter at this sort of event.

A chief concern, it seems, is something called “densification,” which is a fancy term to mean that companies, law firms, banks and residents are settling for less space than they once occupied. The local lexicon is full of talk about “micro” apartments and office “hoteling,” where workers share desks and common spaces.

There are a lot of explanations for the trend. The sluggish economic recovery has all of us looking to do more with less. The push for environmental sustainability has folks eager to use resources more efficiently. Technological advances are making it more feasible to handle back-office functions remotely while eliminating some physical needs; a law firm doesn’t really need a library full of legal books anymore.

And yet the move to go small messes with our assumptions about how development should proceed, given current employment and population projections. By the downtown BID’s account, just maintaining the status quo could still result in an increase in vacancies. Where once 5,400 workers might be needed to bring the vacancy rate in 2012 down to 9 percent, assuming each worker accounted for 250 square of space, now 9,000 workers would be required if they wind up only needing 150 square feet a person.

You can see where this is headed. Either the demand for buildings will decline, or business groups such as the BID must redouble their efforts to attract even more people and companies to the city — this at a time when the suburbs are hatching their own plans for rival town centers and transit-oriented development.

“Our regional neighbors are catching up,” observed Steven Jumper, chairman of the BID’s executive committee, and director of corporate public policy for Washington Gas.

Downtown has taken impressive strides in recent years, but it sounds like its work is not done yet.