A GOOD EXAMPLE of constructive federal pressure has just been seen in Fairfax County, where the board of supervisors this week withdrew its opposition to a subsidized housing project in order to save a $3.7-million community-development grant. The episode brings home to the Washington area what suburbs elsewhere have already learned: that the Department of Housing and Urban Development is serious abot providing the popular community-development aid only to suburbs that meet their responsibilities in the housing field.

The Fairfax situation was fairly typical. The average cost of a home there is now at least $65,000. County housing officials have estimated a need for nearly 27,000 low - and moderate - income units to serve not only the poor, but alos residents such as teachers, police officers and the lower-level whitecollar workers whose jobs and business the county is tyring so hard to attract. Although county plans required by HUD call for more subsidized housing, most board members tend to back off from specific projects whenever community objections arise.

That was what happened in the case of Rolling Road Estates, a proposed townhouse development near Springfield for 70 moderate-income families (under $16,200 for a family of four) and 30 low-income families (under $10,500). The area's residents made the usual - and, to us, unpersuasive - complaints. So the board rejected the project, at a meeting attended by only five members of the nine-person board.

Two or three years ago, HUD officials would probably not have complianed. But HUD has been getting tougher. In the past two fiscal years, community-development grants have been denied in 27 cases for local failure to fulfill subsidized housing promises. Thus there was reason for the Fairfax board to take notice when HUD's area director warned that rejecting the Rolling Road project would "seriously jeopardize" its aid.

The board's response Monday was not exactly ringing. Supervisor Alan Magazine ddid get the previous vote reversed, but only because two of his colleagues decided to abstain. Even so, the board's new posture of nonobjection keeps the project alive - presumably will save the $3.7 million that the supervisors and their constituents want.

The project's future, though, is not clear. It must now be approved by the Virginia Housing Development Authority, which allocates the relevant federal housing and statewide. The VDHA, while sympathetic to publicly aided housing for the elderly, has not been too hospitable to family-oriented projects that are controversial. It seems to us, though, that the state agency should respect both national policy and local governments' conclusions. Congress and HUD have made it clear that federal housing and community-development aid should be used in tandem. Fairfax County has gotten the message and decided, however grudgingly, to go along. If the state throws up unreasonable roadblocks, more federal leverage may have to be applied to keep the programs on course.