The real estate tax-reform picture is beginning to take shape here, weeks in advance of so-called "Treasury-II" -- the second version of the Reagan administration's blueprint for overhaul of the federal tax system.

Discussions with members of the congressional tax committee and Treasury staffs point to several practical guidelines for anyone interested in buying, selling or financing real estate this spring and summer.

Guideline number one: Don't sit around biting your nails over what Congress and the White House are going to do to vacation homes, limited partnerships, depreciation schedules, cut-rate home mortgages financed with tax-exempt bonds, or generous tax credits for fix-ups of historic real estate.

Act, don't fret. Whatever you invest in during the next eight months is virtually guaranteed to be "grandfathered" -- held harmless -- under comprehensive tax-reform legislation.

To wit:

* Second homes bought this year under current favorable tax-code provisions allowing unlimited mortgage and property-tax deductions won't have the rules changed on them next year, even in the unlikely event that a tax-reform package sails through Congress this summer. Current buyers and owners will be protected, although future buyers will not.

* Investment properties purchased and placed into service this calendar year will continue to qualify for 18-year depreciation for the duration of your holding period, even if Congress extends depreciation terms far beyond 18 years beginning next January.

Grandfathering transactions completed this year -- a matter widely misunderstood by consumers and investors nervous about tax reform -- is a political fact. The chairmen of Congress' key tax-writing committees, Rep. Dan Rostenkowski (D-Ill.) of the House Ways and Means Committee and Sen. Bob Packwood (R-Ore.) of the Senate Finance Committee, affirmed it publicly March 15.

Expressing concern that taxpayers' uncertainty may have "adverse effects on business and investment decisions," the chairmen said that "any changes contemplated in the current tax-reform proposals will be prospective in their application."

In other words, there will be no ex post facto tax changes. If you buy a house or invest in a partnership in 1985 to obtain tax benefits allowed in the code, you are safe. Under no circumstances will Congress take away your tax benefits, so long as you qualified legally for them this year.

Guideline number two: Don't bank on any wholesale softening of real estate tax-reform provisions in the second Treasury package coming next month. There is likely to be some backpedaling on depreciation terms for investment real estate (the 63-year standard in the original plan was too tough). Sources say the originally proposed indexing of mortgage-interest income also may be dropped or modified, as may the proposal to tax as corporations all partnerships with 35 or more members.

But beyond that, "don't expect any manna from heaven" to drop on real estate, said one committee official working closely with the Treasury. Virtually all the controversial proposals designed to slash real estate tax advantages -- from cutting out local property-tax deductions to killing tax-exempt home mortgage bonds -- will be back for the big debate.

Guideline number three: The odds still weigh against enactment of a comprehensive reform bill this year, but are far better for something less ambitious -- a "loophole closer" late in the year. It would raise tax revenues, cut real estate tax benefits where their lobbyists are weakest and reduce taxpayers' tax brackets by a notch, all in one package.

The loophole-closer scenario assumes that, despite brave talk from Treasury and congressional leaders this spring, their realistic chances for enacting a major reform package in 1985 will disintegrate by the heat of the summer. (Even with the Great Communicator pushing with full force, complete overhaul of the tax system is too big a pill for Congress to swallow in the span of just eight months.)

Once that fact sinks in, the true tax ball game of 1985 will take shape for real estate: how not to pay a disproportionate share of the national price for cutting the federal deficit. Depreciation, second-home mortgage deductions, investment tax credits, tax-shelter partnerships and tax-exempt mortgage financing all will be on the chopping block this fall -- piece by piece, dollar by dollar, but not as part of a grand movement for tax-code simplification.