The Democratic chairman of the House Ways and Means Committee yesterday outlined a one-year alternative to the Reagan administration's three-year tax cut bill which he said "preserves the spirit of the president's . . . plan," but which the president promptly denounced as "less than half a loaf."

The proposal would cut individual taxes by $28 billion next fiscal year, and give $12 billion in savings incentives and business tax cuts. President Reagan's plan would cut personal taxes by $44 billion in fiscal 1982 and business taxes by $10 billion.

Chairman Dan Rostenkowski (D-Ill.) said in a speech in Chicago that he was "defining the scope" of a bill that he believed could win Republican as well as Democratic support in Ways and Means. Ranking Republican Barber B. Conable Jr. (N.Y.) responded quickly that the plan was "not a consensus bill," saying that he and the 11 other Republicans on the committee would stick with the president's proposals.

But Republicans may find it difficult to vote against many of the Rostenkowski recommendations, many of which have been proposed by Republicans in the past. Reagan wants to cut individual income tax rates across the board, and for business, to increase depreciation allowances. Rostenkowski suggested instead:

A cut, effective next January, in the so-called marriage penalty whereby two-income couples pay more in taxes if they are married than they would if single.

An immediate cut from 70 to 50 percent, effective July 1, in the maximum tax rate on unearned or investment income.This would also bring the maximum capital gains tax down from 28 to 20 percent.

Liberalization next year of the rules for individual retirement accounts (IRAs), under which income can be protected from taxes if it is saved. Among other things, Rostenkowski suggested extending this form of forgiveness to persons covered by pension plans, who are now excluded.

An increase, effective July 1, in the so-called zero bracket amount or tax threshold below which income is exempt from tax. This particularly helps the working poor.

Cuts, effective July 1, in marginal rates, to help those in the $20,000 to $50,000 range.

Special tax relief for unprofitable companies -- autos might be an example -- possibly by letting them claim additional refunds from the Internal Revenue Service for taxes paid in the past.

Rostenkowski, who has had extensive discussions with committee members in the last few days, said that the plan has "enough support among Democrats and Republicans to pass" the committee, although aides said they did not want to claim that everybody had signed off on it. Conable denied that any Republicans would support it rather than the president's plan. d

Sen. Bob Dole (R-Kan.), chairman of the Senate Finance Committee, called Rostenkowski's suggestion a "constructive addition to the tax cut dialogue." But he called for a larger multiyear tax reduction program rather than "a one-year 'shot-in-the-arm' tax cut."

Ironically, Republicans are now apparently opposing immediate action on tax measures they have supported for years. Dole pointed out that "most [of the measures] have been sponsored by Republicans" and listed "the capital gains reduction, the IRA expansions, the research and development credit, marriage penalty relief, and the easing of the taxes of Americans working abroad" as appealing.

Democrats have charged that the president's plan helps the rich more than middle- or lower-income groups. Rostenkowski, while endorsing the administration's goals of more savings and investment, said "the collective effect of these [personal] cuts is focused most directly on people earning between $20,000 and $50,000" with some additional aid for the working poor.

But the proposed top rate cut would cost $3.3 billion in fiscal 1982 and overwhelmingly benefit the rich. Two-thirds of the cut goes to those with incomes of more than $200,000 a year. Treasury Secretary Donald T. Regan yesterday attacked the proposed cut as "inequitable," although he has in the past supported it, and said that it should be cut over three years with other rates.

On the business side, Rostenkowski promised special aid for small business, which could include a cut in the corporate tax rate, and set out standards for a change in depreciation rules which would require some alterations to the president's 10-5-3 plan. For example, 10-5-3, in combination with a more generous investment tax credit, gives more than a dollar of tax relief for each dollar invested, according to many economists.