If you own real estate and are worried that President Reagan's ringing State of the Union endorsement of tax reform could cost you big bucks in 1985 or 1986, take heart.

No one is shouting from the rooftops about it yet, but the political pendulum here has begun to shift perceptibly in your favor. Conversations with tax experts on Capitol Hill, the Treasury and industry groups last week point to important changes under way in the prospects for major reforms.

Here's a quick overview:

* The President's rallying cry on tax simplification notwithstanding, the timetable for full congressional consideration and action on any comprehensive reform package is slipping steadily, and now probably extends beyond 1987. Not only has the Reagan administration decided to put tax changes on a slower track than originally contemplated, but so have key members of Congress.

* Hard economic evidence that some of the Treasury's proposals would force tenants of all incomes to pay drastically higher rents -- particularly for the poorest families -- is beginning to change minds on Capitol Hill.

"If tenants are going to be slapped with 40 percent higher monthly costs simply because Treasury wants to clean up the tax code," said the legislative assistant to one key senator, "then that really changes the political ball game. I don't think we want to cut people's income tax brackets by a few percentage points just to turn around and hit them with intolerable rents."

* The arrival of James Baker as secretary of the treasury has brought a new openness to critical assessment of the original reform blueprint unwrapped last December. Baker has made clear in discussions with Senate and House tax committee leaders that the December package is a starting point, and that any provisions that would send the American economy backwards should be dropped.

The White House won't even be sending precise legislative language to Capitol Hill in its tax-simplification message due late this month. Tax reform as a concept has the presidential seal of approval, in other words. Political hot potatoes within the reform package, by contrast, haven't even been touched by presidential fingers -- nor will they in some cases for months to come.

* The war chests and research studies being assembled by the nation's real estate lobbies to fight the Treasury package are unprecedented. Their size and scope will at least delay -- if not cripple -- congressional action on issues like depreciation, second-home interest write-offs, partnership taxation and tax-exempt financing.

Every major realty-interest group in the capital has either collected money from its members to fight the Treasury, or is in the process of doing so. The National Realty Committee, for instance, has raised and put aside $250,000 just to pay for econometric studies by the Price Waterhouse accounting firm and researchers from Princeton University. Preliminary results from the studies suggest that rents in commercial office buildings -- the committee's main interest -- would jump at least 25 percent if the Treasury plan were adopted.

Other studies are under way or have been completed by the National Association of Home Builders, the National Association of Realtors, the American Land Development Association, the National Apartment Association and a coalition of groups seeking to preserve development incentives for lower- and moderate-income housing.

The National Association of Home Builders' econometric study illustrates the types of conclusions Congress will be bombarded with later this spring. Loss of the current set of tax code incentives will raise rents in apartments from 30 to 60 percent, according to economist Michael Carliner, the association's vice president for housing policy.

"What we see flowing out of the Treasury tax reforms," said Carliner, "is a very scary period of social disruption that will later cost Congress substantial budget dollars to remedy."

That's the message -- overblown or not -- that armies of lobbyists will be delivering on Capitol Hill throughout 1985. The odds that it will get through to the White House, at least in part, are on the rise at the moment.