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WASHINGTON - Congress and state and local governments may want to think a long while before giving tax incentives to promote better housing and urban renewal. But they really shouldn't have to think too long before ending some of the present tax incentives that promote poorer housing and urban decay.

A growing body of experts believe present tax systems operate exactly that way. The Federal villain is the income tax law, and particularly its depreciation provisions. The state and local villain is the property tax, and the way it's assessed. Together, in the view of many urbanologists, these operate to discourage building, inhibit improvement of existing structures, stimulate rapid and repeated turnover of slum properties, and keep vacant land off the market and drive up the cost of developed land.

The property tax, on which local governments depend for almost nine of every ten tax dollars, to already the target of common criticism, to be sure. Assessed valuations vary all over the lot, often without obvious* reason. Religious, charitable, fraternal and other organisations gain too-easy exemptions. Many assessors are untrained, and subject to political pressures. Tax rates are already higher in the impoverished central cities than in the suburbs, speeding industry's outward flight.

Less clearly recognized is the way the property tax operates at exact cross-purposes to some of the ends cities want to achieve. A recent symposium sponsored by the National League of Cities concluded that "too often, it makes it more profitable to misuse and under-use land than to use it wisely and fully, more profitable to let buildings decay than to improve or replace them."

Since most property taxes are based on market value, new property is usually assessed more heavily than old, improved property more heavily than run-down. If a property owner builds an apartment or office building on unused land, remodels a run-down tenement, adds a wing to an apartment house or modernizes a factory loft, his property taxes immediately go up to reflect the new value of the improved property. For many owners, it's a lot easier - and a lot less costly - to leave the property as it is: "Heavy taxes on improvements," says the National League of Cities, "are bound to discourage, inhibit and often prevent improvements."

Most owners worry over still another tax result: Improvements often call the assessor's attention to the fact that the property was undervalued for tax purposes all along. Slum properties in many cities are assessed at a tow figure in relation to their actual earnings; the assessment may be based on the shoddy appearance of the buildings or the deteriorating neighborhood, rather titan on the income produced. The tax man may decide to raise the assessed value of the original property at the same time he adds on the value of the improvement.

In almost every city, moreover, housing, offices and other buildings are taxed far more heavily than vacant land - a custom dating to the days when most Americans were farmers. Low taxes make it easy for owners to bold land off the market, particularly in downtown areas - letting it stand idle or using it for parking lots. They can count on rising land prices not only to repay the taxes paid each year but to yield a substantial extra profit on eventual sale.

This hoarding drives up the cost of the land that is for sale. It adds to urban sprawl, as builders are forced farther out into the country for land they can afford.

One possible answer to the property tax problem is tax abatements for remodeling or other improvements - a refund of former taxes, a credit against taxes owed on other income, or merely the promise of no higher taxes for a specific number of years. These, of course, are all tax incentives. Another answer increasingly advanced is to assess land at a far stiffer rate, closer to the rate on buildings or even higher. Pittsburgh taxes land at double the rate used for buildings, and some experts think the city's extensive facelifting is due in good part to this policy. In 1962, Southfield, Mich., doubled its tax on land and reduced its tax on improvements, and since then it has enjoyed a mammoth building boom.

Both the National Commission on Urban Problems, headed by former Sen. Paul Douglas, and the President's Committee on Urban Housing, headed by Edgar Kaiser, have recommended that states and cities consider shifting more of the tax toad onto the land. "Lighter taxation of buildings," the Kaiser committee said, "might remove existing tax disincentives which discourage new construction, rehabilitation, or adequate maintenance of housing."

The Federal tax laws Jeopardize urban upkeep in somewhat different fashion. For one thing, they provide that the coat of "repairs" can be completely subtracted from taxable income in the year they are made, as a current expense, while the cost of "alterations" or "additions" must be counted as a capital investment and subtracted only gradually through annual depreciation deductions. Some property owners see this as extra reason to settle for minimal repairs rather than major improvements of run-down property.

In addition, the depreciation schedules themselves tend to encourage turnover of slum properties. The tax laws permit extra-heavy depreciation deductions in the early years of ownership, substantially reducing the taxes on the income the property produces. Many owners take advantage of these large deductions for 10 or 12 years, then sell the property (at advantageous capital gates rates on any profit). The new owner can begin the depreciation process all over again on his investment, while the former owner uses tee sale, proceeds to buy another property and start the depreciation process there. This, too, makes for minimum upkeep outlays rather than substantial efforts to upgrade the property for long-haul holding. ' Here again various answers are possible. Some students would follow the tax incentive route and permit current-year deduction of money spent on major rehabilitation and improvement. Others would omit total depreciation recoverable on any property to the original cost plus the costs of subsequent improvements - denying an endless tax deduction for owner after owner. Still others would withdraw depreciation allowances for buildings that fail to meet local health and safety standards.

Obviously, it's politically more popular to grant new tax benefits than to take away existing ones. Yet it's entirely possible that the latter course may do more for the city and its people.