TO MANY PROPERTY owners and investors, having a building declared historic used to be no blessing. The designation brought little gain, except perhaps a plaque to put on the wall, and could restrict the owner's freedom to change the structure or tear it down. But preservation has become much more popular since 1976. That's when Congress changed the federal tax code to provide substantial incentives for rehabilitating historic buildings for commercial purposes.

Although the eligibility rules are cumbersome, the law is already having worthwhile effects. For instance, it has spurred the renovation of the Chrysler Building in Manhattan and helped developers rescue the Bellevue-Stratford Hotel in Philadelphia, which had been a financial victim of the outbreak of Legionnaires' disease there in 1977. Hundreds of less famous but equally worthwhile hotels, office buildings and industrial structures across the country are also being revived. Many of these are being converted for housing or retail activities as part of a rejuvenated "old downtown." There is some question about what will happen to some of these projects after their developers reap the accelerated benefits that the law provides. In general, though, this part of the program seems to be working well.

Yet just as some investors are rushing to get into the preservation field, others are fighting to keep their properties off the historic lists - because the tax law also denies some benefits to owners who demolish historic structures and build something else. The strongest objections to this have come from the Procter & Gamble Company, which is trying to preserve its options regarding its Ivorydale plant in Ohio, built in 1837. Besides fighting the designation of the factory as historic, the company has complained to Congress that the tax law is unconstitutional because it could deny the owners of historic properties some tax benefits without their consent.

That is a flawed line of argument. A number of court cases have established that governments do have the power to encourage or even require preservation of structures or areas of public importance, regardless of the owner's preferences. As historic designation comes to carry more financial weight, those decisions do need to be made more carefully, especially where industries are involved. Yet the owner's desire to demolish or renovate should not decide the case. The controlling factor should be the public benefits and costs.