'KEEP Out."Until last month, that was the invisible sign posted around the exclusive suburban community of Chester, N.H. Highly restrictive zoning laws have kept the town's population down to 2,300 and barred developers from building low- and moderate-income housing. Now, in what is being hailed as a landmark decision, the New Hampshire Supreme Court has ruled that towns cannot use so-called snob zoning laws to prevent the development of low-income housing. The decision draws in part on a precedent established by the New Jersey Supreme Court in the mid-1970s. Every community, according to the New Hampshire court, is obligated by law to permit a fair share of housing for people of moderate and low incomes. It decrees that towns "may not refuse to confront the future by building a moat around themselves and pulling up the drawbridge." Town selectmen deny that Chester's zoning code was deliberately exclusionary. Their intention, they say, was to preserve the town's rural nature. But even if their motives were benign, the effect was to deny less affluent outsiders an opportunity to share the benefits of suburban living. Controlling growth and preserving the essential character of a community are legitimate goals. But affordable housing has become an oxymoron in many suburbs, especially where rigid zoning codes prevail. The New Hampshire high court's ruling need not serve as an invitation to helter-skelter development. With proper coordination between planning boards and developers, a more egalitarian mix of housing can enhance, rather than hurt, the reputation of a suburb. Good fences make good neighbors, according to Robert Frost. But snob zoning laws make bad fences - and allow for no new neighbors at all.