AS A GOOD cause, preserving neighborhoods has come to rank right up there with protecting families, conserving energy and promoting grassroots government. Thus when Congress recently passed a study-commission law called the "National Neighborhood Policy Act," nobody quarreled with the premise that "existing city neighborhoods are a national resource to be conserved an revitalized wherever possible."

That language is worth some attention, though. To start with, it reflects a major change in urban polities and policies, away from massive public redevelopment and the concentration of private resources in rich and growing areas. Instead, the "neighborhoods" approach emphasizes non-disruption, social stability, attention to the working class, small-scale programs and grass-roots organization - often, at least initially, in opposition to city hall.

Beyond that, however, key terms are largely undefined. What is a "neighborhood"? How can a cohensive community - not just a place - be "conserved" and "revitalized"? What's "possible"? The answers are likely to differ in Washington, Cincinnati and Baltimore; indeed, the answers in Adams-Morgan and Anacostia are not entirely the same. That's not a defect. On the contrary, it is a major reason why the "neighborhoods movement" has come to encompass so much: working-class parishes threatened by blight, wealthier area fighting high-density development, ad hoc citizens' campaigns against crime, and highly structured programs for rehabilitating homes.

The localism and diversity in all of this are, ironically, the elements most endangered by success. So far, the "neighborhoods" concept has followed the path of most modern protests that catch on. Activists in scattered communities have formed coalitions. Ideas and experiences have been shared. Foundations and public agencies have gotten interested. Studies have been funded. Conferences have been held. Support has been amassed in Congress and the executive branch. All this is fine, if slightly faddish in some respetcts. Now, hwever, there is to be a "national neighborhoods policy." And with that, if things follow the normal course, with come federal funds and rules and definitions and the rest, gradually smothering the creativity and citizens' initiatives on which the whole idea was founded.

Fortunately, many "neighborhoods" advocates see the danger. They remember all too well what happened to community action and how the model-cities program was suffocated by its own intricacy. They seem to recognize, too, that no national agency can prescribe specific programs - much less local political arrangements - for every neighborhood in the land. Thus the new law sets up a study commission, composed primarily of local activists, to spend one year and $1 million studying the whole field and shaping recommendations, primarily on financing and taxing policies for existing residential areas.Some good ideas may come out of this. More important, it gives everybody some time to think about how, or indeed whether, federal programs can foster neighborhood action without trampling the grass roots.