After a months-long buildup, Prime Minister P.W. Botha tonight outlined plans to modify South Africa's strict apartheid system of racial segregation by giving political representation in separate parliamentary chambers to the country's 2.5 million persons of mixed race and 800,000 Asians.

The nation's majority of 21 million blacks does not figure in the proposals announced today, except at the local-government level.

Botha chose the more conservative alternative put to him in May by his President's Council, which left open the question of whether the new Parliament should consist of separate chambers or a racially mixed house.

Political commentators here believe Botha rejected a single house because he is worried about the loss of Afrikaner supporters to the new right-wing Conservative Party led by Andries Treurnicht, former leader of the ruling National Party's powerful Transvaal branch.

The three-chamber idea was approved by the National Party in 1977, and Botha's return to it now will make it difficult for Treurnicht to continue assailing him as a dangerous reformer.

At the national level, blacks are restricted to exercising their political rights as citizens of one of 10 small tribal homelands. Four of these have been given independence, but that status has not been recognized internationally.

Speaking here at only the third federal congress of all the National Party's provincial branches, Botha told 3,000 delegates that they were assembled for "an appointment with the future."

South Africa must change to avoid "chaos and a cul-de-sac," he said, describing his proposals as a formula that will "bring about justice without endangering the whites' sense of security."

Botha also announced acceptance of a President's Council recommendation that, instead of a prime minister, South Africa should have a strong president holding wide powers and not responsible to Parliament.

But he rejected the idea of a Cabinet outside Parliament, which had also been suggested. He said the president should be able to pick his Cabinet from inside or outside Parliament, and include mixed-race persons, who are known in this country as Coloreds, and Asians as well as whites if he wished. Botha said the president should be elected by an electoral college of 50 whites, 25 Coloreds and 13 Asians from the three chambers.

This will ensure that the president will be white and a member of the National Party, which has a huge majority in the white Parliament.

Botha outlined a complex formula for the working of this three-chamber Parliament. A distinction is to be made between matters of "communal concern"--defined as "matters which are considered by a group as part of its identity," such as religious worship, education and care of the aged--and matters of mutual concern to all groups.

Legislation on matters of communal concern will be passed by the chamber of the community concerned, while laws on matters of mutual concern will be passed by the three chambers sitting separately. The president will decide which matters are of communal concern and which are of common concern.

If there is a conflict between the three chambers on a matter of mutual concern, the multiracial President's Council will make "an authoritative finding."

Botha will seek an expected endorsement of his policy from the congress Saturday, then take it to the provincial congresses, the party's official policy-making bodies. It must also be enacted by the present all-white Parliament. Botha said tonight it could take several years to put into effect.