A blue-ribbon commission appointed by Prime Minister Yasuhiro Nakasone is helping make respectable an idea that bordered on heresy a few years ago -- that building prosperity by single-minded maximization of exports is an outmoded, even dangerous approach for Japan, and must be scrapped.

The commission's report, officially still confidential, is to formally go to Nakasone next week. In legal terms, it represents nothing more than the views of its 17 members. But when change comes in Japan, it is often preceded by a plea from such a prestigious panel to add legitimacy.

Nakasone has given the commission's work enormous buildup when meeting in recent months with critical foreign visitors. He has said it will guide his policy formulation.

The report has been a rush job, partly because the prime minister ordered that it be completed before he hosts President Reagan and five other leaders of the western industrialized world at a three-day economic summit that convenes here on May 3.

His timing is political, an effort to shift the spotlight of international criticism away from Japan at the summit. It is hoped here that the report will be taken as evidence that Japan is doing something about its mammoth surpluses -- $50 billion on the current account in 1985.

But the commission's conclusions do reflect a feeling that is gaining credence in policy-making circles here: that Japan has outgrown its post-war economic approach, and that keeping it will only perpetuate potentially ruinous tensions with its major trading partners.

"This is an historical turning point for the Japanese economy and the people's life," said Takashi Hosomi, a commission member who is chairman of the Overseas Economic Cooperation Fund, the agency that administers Japan's foreign aid.

Statements like that will be true only to the extent Nakasone succeeds in implementing the plan. The real test will begin when he takes it to the legislature, ministry offices, business federations and the legions of special-interest groups that will have to get on the team if it is to work.

Some of that "consensus-building" is supposed to have taken place already. The commission comprises a cross-section of government, business and academic people and has held close consultations with groups on the outside.

It was headed by Haruo Maekawa, former head of Japan's central bank. Working under the unwieldy title of Advisory Group on Economic Structural Adjustment for International Harmony, it has been drafting a report since the fall, with Nakasone sitting in on its formal meetings.

The committee's keystone is the assertion that Japan, now the world's third-largest economy, must base growth not on exports, but on the buying power of its 120 million citizens. To do this, it must inflate domestic demand in a country known for a tight-fisted reluctance to enjoy the fruits of its labor.

Few of the ideas presented in the report are new. But in many cases, they have a form and specificity lacking in previous "packages" put out by the government. The major points would:

*Increase private housing construction by a variety of incentives involving taxes and deregulation.

*Promote new public works projects by local governments.

*Stimulate private buying through wage increases, tax cuts and institution of a five-day work week (millions of people in Japan work 5 1/2 or 6 days).

*Review (which can be a code word in Japan for change) the country's system of tax-free savings accounts.

These are all intended to make the Japanese people consume more of what they make, which would leave less to be shipped abroad.

Other steps look at increasing imports and integrating the country's notoriously inbred economy into the outside world's:

*Phase down noncompetitive industries that have been shielded by protectionism, such as coal mining and other ailing sectors. As they went out of business, the differences would be picked up in imports.

*Lower barriers that prevent imports of certain agricultural commodities.

*Increase imports of manufactured goods.

*Encourage foreign investment in Japan and Japanese investment overseas.

*Improve market access for foreign products through such steps as stricter enforcement of antimonopoly laws, simplification of standards and monitoring of awards of international contracts.

In addition, the plan calls for the stabilization of exchange rates, further liberalization of financial markets in Japan, including creation of a short-term securities market in Japan.

Finally, the blueprint would have Japan continue to increase its foreign aid, work to overcome the Third World debt crisis, and integrate itself into the world economy, taking a role in forums such as the General Agreement on Tariffs and Trade that is "commensurate with its international status," a phrase often heard here.

On many points, the report pulls its punches when confronted with political obstacles that the Japanese like to say are insurmountable.

U.S. officials here are still evaluating the plan, which is not known in all its details. But one said that based on information available, it reflected much of what the United States has been recommending.