Washington office developers and landlords are salivating at a glut of law firms whose leases expire in 2016 and 2017.

For every law firm that signs a lease in a new building, another landlord is left with a vacancy, however, and the needs of law firms are changing fast enough that filling the departed space is becoming more difficult.

“It’s the biggest challenge we’ve got as office developers,” said Raymond A. Ritchey, senior vice president at Boston Properties.

Speaking on a panel held by the D.C. Building Industry Association last week, Ritchey said every lease he has signed with law firms in recent years has been for smaller space than the firm occupied previously.

This is partly a result of changing organizational structures at the firms, said Tom Fulcher, executive vice president at the brokerage firm Studley, which represents many of the area law firms. Law firm offices that were designed in the 1980s or 1990s, he said, typically provided space for an administrative assistance for every one or every two partners; now one for every four is the norm. As firms shift, the extra desks sometimes either sit empty or are used for storage.

“The ratio of attorneys to secretaries has changed pretty dramatically,” Fulcher said.

Also disappearing are law libraries and even the grand corner offices for senior partners, Fulcher said. “Now what you’ll have is the offices are all the same size and the corner a lot of time will be conference space,” he said.

A new focus is on delivering natural light to as many of the interior offices as possible, often with dramatically increased use of glass. Smaller conference rooms and shared work space sprinkled around the building have become preferable to large conference rooms that were popular decades ago.

For developers and landlords with the older designs, however, shifting to meet the new demands can be expensive. Robert H. Braunohler, regional vice president at Louis Dreyfus Property Group, said at the DCBIA event that developers looking to land law firms years ago aimed for office depths of 120 feet to accommodate space for administrative support staff and interior rooms.

“Law firms are looking for 60- to 90-feet depth now. So it’s a very different type of building,” he said.