THE DEPARTMENT OF HOUSING and Urban Development has been reminding local officials recently that community-development block grants come with strings attached. The message is aimed especially at middle-class suburbs, which relish the federal aid for sewers, parks and other facilities but resist providing housing for low- and moderate-income people. Central cities, however, are not home free; HUD Secretary Patricia R. Harris and Assistant Secretary Robert C. Embry have told mayors bluntly that they will also have to concentrate their CD programs in poorer areas, instead of scattering projects all over town.

If HUD's new toughness comes as a jolt to some local governments, it is only because the Ford administration never consistently enforced Congress's 1974 directive that the $4-billion grant program be used primarily to benefit lower-income people, expand their housing opportunities and prevent or eliminate blight. According to the National Association of Housing and Redevelopment Officials, local governments actually used 49 per cent of the first-year grants and 56 per cent of the second-year funds in higher-incomes areas.

No more. HUD is now reviewing jurisdiction's housing-assistance plans and performance much more carefully. As staff writer William Chapman reported the other day, an application by Hempstead, N.Y., was turned down recently because of the community's poor record on low-income housing. Last month the city council of Boca Raton, Fla., hurriedly approved a subsidized housing project in order to save a $400,000 grant.

HUD's new aggressiveness is not going to bring widespread, instant economic integration to suburban America.For one thing, suburbs may opt out of the program if they decide that exclusivity is worth the price. A number have done so, including Cicero, Ill., and Warren, Mich., whose resistance to subsidized housing caused so much trouble for former HUD Secretary George Romney some years ago. Moreover, even where suburbs are amenable to aiding lower-income residents and workers - as several Washington-area jurisdictions are - progress is likely to be slow. The high cost of housing not only keeps poorer people out of the private markets; it also makes subsidies very expensive and restricts their scope.

Even so, the CD program can be an important catalyst, especially if coordinated with other efforts such as enforcement of fair-housing laws. If HUD hangs tough, local officials will no longer be able to evade politically sticky housing issues so easily. Indeed, federal firmness may give some timid local governments sufficient reason or excuse to accept, finally, their responsibility for helping to enlarge housing opportunities. As a result, both suburbs and cities can gradually become more economically and residentially diversified, and the isolation of poorer urban dwellers will be reduced. That's what the program is meant to promote. For taking it seriously, despite the likelihood of unpopularity in some quarters, Mrs. Harris and Mr. Embry deserve congratulations and support.