If you're one of many Americans who own and invest in real estate, you may be surprised to learn that Congress is on the verge of passing a $900 million federal tax-increase bill aimed at property owners.

With all the distractions of hostage crises and July 4 festivities, no wonder you hadn't heard the news. But here are the facts: Once the Senate and House resolve the minor differences between their respective pieces of legislation -- probably in mid-July -- the 18-year standard depreciation period for real estate will be increased to 19 years. The increase will be retroactive to May 9 for all investment properties "placed in service" on or after that date.

In other words, if you happened to purchase a rental condominium or duplex May 10 and began leasing it out in June, you cannot use the 18-year write-off schedule that you'd planned on. You've got to depreciate it instead over 19 years or face federal tax penalties -- despite the fact that the change was retroactive, you hadn't the foggiest idea it was coming and you would have paid a lower price if you'd known.

Sorry, but those are the breaks of the bare-knuckles tax games under way on Capitol Hill this year. The extra year being tacked onto the standard depreciation formula -- on the heels of 1984's increase of three years -- is a $900 million payment designed to end real estate's legislative hostage crisis.

The $900 million is the estimated amount that American property owners and buyers will pay the Treasury over the next four years as the result of the depreciation change.

In return for the $900 million, the Treasury and congressional tax-committee leaders have agreed to release broad segments of real estate from coverage under the controversial "imputed interest" seller-financing tax-code changes enacted last summer.

Assuming the House and Senate pass the bill and President Reagan signs it, the new federal rules on imputed interest will work as follows:

When a seller helps finance the sale of his property and the total amount of the seller's financing is less than $2 million, the loan will be able to carry an interest rate as low as 9 percent without incurring Uncle Sam's tax wrath.

If the loan contract rate is below 9 percent, then the Internal Revenue Service will be empowered to assign, or impute, a higher rate equal to the government's so-called AFR, the applicable federal rate. The government's three AFRs, based on the current Treasury costs of short-term, medium-term and long-term capital, range up to 10.62 percent at present.

(By the way, should interest rates in the economy overall continue to fall, the new legislation will permit seller-financers to lower their minimum loan rates as well. It allows sellers to choose the lesser of 9 percent or 100 percent of the appropriate short-, medium- or long-term AFR rate.)

Let's say you're selling a $300,000 rental property, for example. Under the new tax-law revisions, you'll be able to offer potential purchasers a 9 percent first or second mortgage or deed of trust. Under the original version of the 1984 tax law passed last summer, by comparison, the IRS would have had the power to tax you as if your seller note bore a rate of 12 3/4 percent or less.

Buyers and sellers of larger-scale real estate will have to operate under somewhat different rules. When a transaction involves more than $4 million worth of seller financing, the seller's note will have to carry a rate equal to the appropriate short-, medium- or long-term federal rate. A 10-year, $5 million purchase-money note on an apartment building, in other words, would have to have a minimum rate of 10.62 percent -- the government's applicable minimum figure for long-term notes this month.

Transactions involving between $2 million and $4 million in financing will be subject to a "blended" minimum-rate formula, the recipe for which has yet to be worked out by Senate and House negotiators.

Other key items in the forthcoming real estate-hostage tax act of 1985:

*Property sales that include assumptions of existing below-market-rate loans will not be subjected to the new imputed-interest standards as they would have under the 1984 tax law in its original form.

*Sellers of farms, ranches and closely held businesses probably will be given special blended-rate treatment on sales up to $9 million, but the final formula is uncertain.

*The effective date of all the imputed-interest tax changes will be June 30, 1985. That was the statutory trip-wire deadline beyond which the controversial 1984 tax act provisions would have become the law of the land.