LAST JULY, when Marion Barry was campaigning for mayor, he stood on Bates Street -- in the heart of a Northwest slum area, lined with burned-our and boarded-up rowhouses -- and made a promise to "take the boards off" and open more housing for low-to-moderate-income families in the city. The people of that neighborhood and others like it had heard that kind of talk before -- going back to the last 1960s, when various government rehabilitation plans were announced. But this week, Mayor Barry was back on Bates Street with some specifics: money commitments, a report on what wpuld be done where and -- best of all -- some deadlines for doing things. As the mayor himself noted, it is a most ambitious undertaking. Will it work?

So many deadlines have blown by in the past without tangible progress that it is impossible to be sure. But the plan looks promising. It is a $35-million, government-private program to rehabilitate 733 of the estimated 4,500 vacant, boarded-up units in the city, including areas affected bythe 1968 riots. Perhaps the most encouraging aspect so far is Mr. Barry's report that private lending institutions have pledged $25 million in loans; the mayor further notes that developers and sponsors already have been selected for most of the projects. As for timing, the plan calls for all 733 units to be ready by November 1980. They are to include a little bit of everything in housing -- rentals, condominiums, cooperatives, public housing and 200 single-family houses.

Most of the units are to go to low-to-moderate-income families, officials say, with families displaced by urban renewal getting first priority. But some are to be sold at market rates, to create "economically integrated communities." Certain properties will be part of the city's "homesteading" program, which allows families to buy homes for as little as one dollar if they will fix them up; other families will be able to borrow up to $11,000 for downpayments. In addition, says the mayor, most of the rehabilitation is to be done by minority contractors.

Even if all of this is accomplished on time, of course, the city's housing plight will not have been solved. Officials estimate that the city owns more than 1,000 vacant or partially occupied units, and that there are perhaps 3,500 privately pwned vacant units. As for people in need, there are nearly 7,500 on the public-housing waiting list -- and thousands more seeking improved housing at affordable prices. Can the city make a dent on those statistics? Will the government really be able to find those displaced families to put them at the head of the line? Mayor Barry and his vigorous new director of housing and community munity development, Robert L. Moore, say they are determined to do the job. They are off to an impressive start.