Congress is at least six months away from passing tax reform legislation. But for tens of thousands of unsuspecting first-time home buyers, 1986's tax bill took effect abruptly on New Year's Day.

As a result, housing and legal experts say upwards of 150,000 buyers are now certain to be denied the cut-rate mortgage money they otherwise would have received for their first purchase of a home in the coming 12 months.

That is one of the sobering conclusions emerging from discussions here last week among 350 housing agency executives, bond market lawyers, investment bankers and developers.

First-time home buyers across the country "have no idea what has happened to them in the past three weeks," said William Witte, head of the San Francisco Mayor's Office for Housing and Economic Development. "Congress hasn't passed legislation, President Reagan hasn't signed a bill," but the tax reform legislation approved by the House in December "now has the full effect of federal law" for those buyers, Witte said. Even members of Congress themselves "may not understand" that the pending tax bill now has the force of law in a key segment of the housing market, he added.

The situation that Witte and other housing experts decried last week is a legal quirk arising from the House-passed tax bill, H.R. 3838. The bill contains a wide variety of curtailments of tax-exempt bonds, including imposition of state-by-state volume ceilings and home-price restrictions on so-called "mortgage bonds" issued by counties, cities and states.

The bonds provided mortgage money at discount interest rates last year for nearly 300,000 first-time home buyers nationwide. The bonds also helped to finance more than 250,000 units of rental housing targeted for moderate-income families, according to estimates by the Council of State Housing Agencies.

In many sections of the country, discount-rate housing-bond programs represent one of the only ways for moderate-income families to buy or rent a home in the jurisdiction where they work.

In high-cost Montgomery County, housing bond financing accounts for nearly half of all housing purchases by first-time buyers, according to local officials. In San Francisco, $50 million worth of bonds issued in 1985 accounted for 85 percent of all new housing financed in the entire city. In many states and counties where the 9 to 10 percent discount mortgages are made available periodically following bond issues, would-be borrowers line up for hours outside participating lenders' windows for a chance to submit an application.

All this would be changed drastically under the House-passed tax reform bill. Its volume ceilings and new income limits would cut single-family mortgage financing by a minimum of 47 percent, according to Keith Rasey, research director of the Washington-based Council of State Housing Agencies.

Rather than the 300,000 first-time buyers assisted in 1985, the House-passed tax reform legislation would slash that to 156,000 or less, he said.

But the congressional tax reform bill only has passed the House thus far. The Senate plans to begin debating its provisions this spring. Final enactment -- if indeed it occurs -- is unlikely before the summer.

Moreover, before adjourning in December, the House and the Senate passed resolutions expressing their desire to delay the effective dates of most provisions of the legislation to Jan. 1, 1987, rather than the Jan. 1, 1986, date contained in the House bill.

Why, then, is the House's tax bill having the effect of a fully enacted law in the housing bond market? Why are housing agencies across the country now effectively closed down and cutting their bond-issue plans? Legal experts assembled here said the answers revolve around the resolutions passed by Congress in December. Neither the House nor Senate resolutions carry language that is binding upon the tax-law writers. Neither resolution changed the pending bill in legal terms.

As a result, in the words of Washington, D.C., bond law expert Frederic Ballard, "Jan. 1, 1986, is the only effective date" that bond market investors can take seriously.

Congressional tax committee staff members confirmed Ballard's assessment. Unless the Senate and House clarify their intent on the Jan. 1, 1986, effective date, housing bond issues will be few and far between for the coming six months. So will cut-rate mortgages for first-time home buyers.