The House of Representatives passed its labyrinthine tax-reform bill before heading home for the holidays, but left real-estate owners, buyers and investors scratching their heads about its provisions. In case you're one of them, here's a guide to what the bill does and doesn't do and where it's headed and how fast it's moving:

Q. Does House passage of the measure make immediate changes to any tax incentives currently used by homeowners or investors?

A. None of the provisions of the bill can have legal effect until the Senate passes the legislation and it's signed into law by President Reagan. Because the Senate could make significant alterations in the bill, and may not even bring it to the floor for a vote until late spring of 1986, real-estate tax changes are at least months away.

Q. What about the Jan. 1, 1986, "effective date" in both the House-passed bill and President Reagan's tax proposals? Could there be retroactive changes, covering the entire 1986 tax year, even if the Senate passes the legislation in late spring?

A. The odds are strong that most provisions of the tax-reform legislation will not be retroactive.

Q. How would homeowners and buyers fare under the House-passed legislation?

A. The bill treats homeowners relatively well. Not only does it preserve current deductions for mortgage interest without limit, it also allows full deductibility of local property taxes -- an important departure from the original Reagan plan. It does, of course, cut the dollar value of deductions for typical homeowners by virtue of its lowering of federal tax brackets. High-income owners who currently write off 50 percent of their mortgage interest and property taxes would write off a maximum of only 38 percent of those expenses under the House bill.

Q. Will second homes and vacation units be hurt under the tax bill?

A. The version of the legislation finally approved by the House leaves current tax benefits for owner-occupied second homes intact.

It also expands the second-home category to include time shares of up to six weeks. But vacation-unit owners who rent them out will be affected by the bill's overall reductions in depreciation write-off incentives. Whereas current owners have enjoyed either 15-, 18-, or 19-year depreciation schedules -- depending on when they acquired their property -- future owners would be shifted to 30-year standard schedules. This would reduce the size of the annual tax write-offs produced by the unit and could cut future prices for such real estate.

Q. Will the new tax legislation eliminate low-interest-rate home mortgages for moderate-income, first-time buyers provided by local and state agencies?

A. No. The bill sets dollar-volume limits on the tax-exempt bonds that provide the money for first-time-buyer housing bonds, but does not kill them, as originally proposed by President Reagan. The decision as to how much bond money to allocate for single-family mortgages will be up to state governments, within an overall state-by-state dollar limitation.

Q. What about real-estate tax shelters? Does the bill signal their demise?

A. High-write-off real-estate tax shelters have been under attack in Congress since 1982. If enacted, the new tax-reform legislation will crimp them further. In addition to reductions in write-offs for depreciation, the House-passed bill also creates a tough new minimum tax designed to ensure that high-income investors do not escape taxation via real-estate and other tax shelters.

Q. Will any major real-estate tax-shelter provisions be retained under the law?

A. One of the most generous of all will be retained, though in slightly slimmed-down form: investment tax credits for renovations of historic real estate. Currently, investors can subtract 25 percent of the cost of certified rehabilitations of historic properties off their annual tax bills. Under the House legislation, the credit would be reduced to 20 percent.

Q. What is the outlook for tax legislation in the Senate?

A. Though the battles will be fierce over real estate and other provisions, the political odds at the moment weigh in favor of Senate passage of some form of tax bill by next summer's pre-election recess. The pressure from President Reagan -- who, during the House tax-bill brouhaha, showed he can knock heads within the Republican Party -- will be intense. So will the political attraction of cutting tax brackets in an election year.