IT'S NEVER EASY for elected officials to approve subsidized housing where community opposition is fierce. But that's what we hope that Fairfax County Board will do Monday in its third go-round over Rolling Road Estates, a proposed townhouse development south of Springfield for 70 moderate-income families and 30 low-income families.

Strictly speaking, the question before the board is whether Rolling Road Estates conforms to the county's general housing-aid plan. The state housing agency is prepared to approve financing for the development unless the board rejects it by Wednesday on those grounds. But more is at stake than just technical tests. The federal Department of Housing and Urban Development has warned that defeating the project could jeopardize the county's $3.7-million community-development grant. And on the other hand, board chairman John Herrity and other opponents have escalated the fight into a general assault on county housing policies and what they see as high-handedness by local housing agencies and HUD.

It's true that some Fairfax agencies did not go out of their way to consult the project's prospective - and predictably nervous - neighbors at an early stage. That political erro certainly helped heat up the fray.HUD's action, however, has been both correct and commendable. The federal aid does have housing strings attached, and it's about time for Fairfax and other suburbs to recognize that in order to keep those grants, they have to do more than adopt fine-sounding housing-aid plans. The plans must be carried out.

Like other Washington area suburbs, Fairfax County has made a start toward offering more housing opportunities for people of modest means. We doubt that most board members really want to abandon that course - not just because $3.7 million or more is at stake, but because they do recognize that the housing problems of residents and workers with below-average incomes are serious and getting worse. It may be possible, as well as politically prudent, to modify the Rolling Road development somewhat and in the future to make greater use of other programs, such as rent subsidies for existing units, which have less impact on any one neighborhood. But we hope the board will not abandon, or fatally undermine, a project that is ready to go.