Uncertainty over Congress' tax reform plans is beginning to have major effects on real estate investing patterns. The uncertainty is stimulating minibooms in certain areas, depressing property values in others and forcing many investors to rethink their ventures.

Discussions with executives of top real estate firms here suggest that dramatic shifts are under way in investment strategies. These changes will create opportunities for sharp-eyed buyers and sellers this spring and summer.

"We've stopped looking at anything in resort areas, anything that could suffer if second homes, condos and depreciation get hit," said Bertram Lewis, chairman of Sybedon Corp., a Manhattan-based leader in the tax-sheltered, limited-partnership field.

Sybedon has been the financial force behind $350 million worth of development projects the past 24 months, ranging from renovations of historic hotels to housing for the elderly.

A large chunk of the capital raised for its projects has come from limited-partnership offerings to upper-bracket investors. All of those partnership units provided high write-offs from investment tax credits, accelerated depreciation and other federal tax code investment incentives. These are prime targets of the Treasury Department's tax reform package heading for Congress.

Sybedon and other such firms see tax reform as a two-edged sword for the coming 11 months.

On the one hand, "we're avoiding deals that we can't complete and sell" to limited-partnership buyers by the end of 1985, Lewis said. On the other hand, Sybedon is selling certain of its current stock of high write-off tax-shelter investments, thanks to buyers' fears of an eventual cutoff.

None of the key Treasury tax changes would take effect until next January, even if enacted this year.

As a result, according to Lewis, "A lot of people -- and we are among them -- are saying to themselves, 'Hey, this could be the last good year for tax shelters. Better hook on to one now, before there is nothing left.' "

For example, Sybedon's tax-credit partnerships in historic areas are selling fast, even at a stiff $100,000 a unit. A $30 million renovation under way on San Diego's historic U.S. Grant Hotel that Sybedon just brought to market is selling at a $5-million-a-week clip. The project offers individual investors a first-year, 25 percent federal tax credit -- subtracted directly from their regular income tax bill -- plus 18-year depreciation and other benefits that could disappear next year.

Other New York-based firms say they're seeing a surge of investor interest in a more exotic breed of acquisition: condominium hotels. An executive at Donaldson Lufkin & Jenrette said hotel condos are shaping up as "the only game in town" for investors worried about Congress' menacing moves against tax-shelter partnerships.

Unlike most "syndications" or real estate limited partnerships, hotel condominiums are sold as individual-ownership units. Companies such as Marriott operate condo hotels for associations of individual-unit owners assembled by Wall Street securities firms.

The unit purchasers acquire legal title to a specific room or suite in a downtown or resort hotel, usually financed with a 90 percent mortgage. Investment credit write-offs for furnishings and equipment are substantial, but the prospects of high daily rental income and long-term capital gains are the key motivating factors for most buyers. Prices have ranged from $80,000 a unit to $300,000 or more, at hotels ranging from New York City's Essex House on Central Park to the San Destin Hilton on Florida's Gulf Coast. Because condo-hotel units are neither partnerships nor second homes, purchasers believe that they provide a solid hedge.

Other emerging investment patterns in the wake of congressional and Reagan administrative tax reforms include:

* The market values of commercial properties and office buildings have begun to decline in some areas as tax-motivated investors drop out of the bidding. Those price declines, in turn, open the door to what could prove to be bargain-basement acquisitions for investors who don't believe Congress will pass a reform package before 1987.

* Savvy investors are demanding -- and getting -- financial guarantees against tax disasters from limited-partnership sellers.

Some transactions include guaranteed compensation to the investor should Congress pull the rug out from under the deal.