Ronald Reagan's massive reelection victory could have important financial consequences for tens of thousands of taxpayers who counted themselves in the president's corner politically: real estate owners, small-scale investors, home builders and realty brokers.

Capitol Hill and Treasury sources familiar with the broad outlines of the forthcoming tax-reform package being readied for the White House agree that:

* Real estate will be among the most dramatically affected sectors of the economy touched by the tax plan.

* Pushing the program hard on Capitol Hill early in the new Congress will be the Reagan administration's number one post-election domestic priority.

* Presidential promises of "no tax increase" notwithstanding, large numbers of individual real estate owners -- including small-scale investors in condominiums, rental houses and apartment buildings -- inevitably would pay higher taxes under the coming tax-reform proposals.

* The dollar resale value of many forms of real property -- particularly those purchased, financed or rehabilitated with current federal tax assistance -- would decline if such incentives were abruptly withdrawn.

As one top congressional tax expert put it last week, "There is absolutely no way that the loopholes and preferences that have accumulated in the federal tax system are going to be cleaned out without hurting real estate. We take that as an inescapable fact of life."

The Treasury's tax package, under intensive development since last spring, won't be unwrapped officially until early December. Government sources confirm that it will propose a modified flat-tax concept -- one that cuts individual and corporate income tax rates while jettisoning or restricting longtime tax deductions and credits.

The Treasury plan reportedly will bear similarities to the so-called Bradley-Gephardt and Kemp-Kasten bills introduced on Capitol Hill last session. The first measure, named for authors Sen. Bill Bradley (D-N.J.) and Rep. Richard A. Gephardt (D-Mo.), would end preferential capital-gains taxation for all investments, eliminate deductions for interest expenses except those for home mortgages, eliminate all nonrefundable investment tax credits, and lengthen the customary depreciation term for investment real estate to 40 years, up from the current 18 years.

In exchange for these and other reforms, Bradley-Gephardt would tax individual incomes at graduated rates ranging from 14 percent to 30 percent. Corporations would pay a flat 30 percent of their taxable income.

The Kemp-Kasten bill, co-authored by Rep. Jack Kemp (R-N.Y.) and Sen. Robert Kasten Jr. (R-Wis.), would tax individuals at a flat 25 percent rate and corporations at graduated rates between 15 and 30 percent. Like Bradley-Gephardt, it would eliminate numerous investment tax credits and preferences, and leave home-mortgage interest deductions unscathed. Unlike the Democratic-sponsored bill, though, Kemp-Kasten wouldn't change the standard depreciation term for property owners.

The problem posed for real estate by modified flat-tax plans such as these is whether their undoubted attractions -- lower standard tax rates and simplification of the federal-tax code -- fully compensate for the loss of decades' worth of tax incentives.

For example, with lower individual tax brackets, would sufficient capital flow into sectors of the economy that traditionally have depended upon tax incentives to attract private investors, such as low- and moderate-income housing?

Would urban real estate undergo the sort of rehabilitation boom currently under way without the 15 to 25 percent investment tax credits for historic and other property recently added to the code? Would the country's large number of vacation homes and small-scale residential rental properties fall in value -- perhaps by 15 to 25 percent -- if interest, property taxes, accelerated depreciation and other deductions no longer sheltered taxable income? Even if current owners were grandfathered, allowed to retain their deductions for a period of years, wouldn't the reforms depress demand for such investments by future buyers?

Questions such as these are beginning to be posed by economists and financial analysts. The preliminary answers have some observors deeply concerned.

Analysts at Sears & Roebuck's Coldwell Banker Commercial Real Estate Services, for instance, recently computed the dollar impact of Bradley-Gephardt on typical investment transactions.

In its current form, according to Coldwell-Banker, Bradley-Gephardt would cut the annual investment return on an actual building now under construction in southern California by nearly one-half. Since the value of the tax deductions available from the structure would be severely diminished, according to the analysis, the price of the land underneath the building would have to be cut by 40 percent in order to provide the same yield to a future investor under a modified flat tax.

Such steep slashes in real estate returns and values -- if confirmed by congressional, academic and industry studies -- could create heavy opposition to tax reform next year on Capitol Hill, Reagan landslide or no.