After Mayor Edward I. Koch instituted his housing plan in the 1980s, New York City created tens of thousands of affordable apartments by renovating vacant buildings in neighborhoods that had been ravaged by arson and abandonment.

More than two decades later, Mayor Michael R. Bloomberg, with far less unoccupied property at his disposal, poured money into preserving government-subsidized and rent-stabilized residences as well as into building some new units.

Now Mayor Bill de Blasio, who made the shortage of housing for lower-income New Yorkers a top campaign issue, is promising to build or preserve 200,000 affordable units over 10 years. In setting such a goal, Mr. de Blasio is seeking to do more than either Mr. Koch, whose effort yielded more than 190,000 units over 13 years, or Mr. Bloomberg, whose push saved or added 165,000 units over 12 years.

It is a lofty goal, experts said, and Mr. de Blasio and his aides said it must be, given the pace of change washing over the city. Apartments affordable to people with low or moderate incomes are being lost to deregulation faster than units can be created or preserved. With federal housing funds diminishing, the city housing authority is struggling to maintain many of its 334 projects, and it has no plans to add more. And much of the new residential construction in the city is not being built with poor New Yorkers in mind.