Nine members of the D.C. City Council joined yesterday in introduction legislation to clamp strict permanent restraints on future condominium conversions in the city and require a majority vote of tenants before any conversion could begin.

If enacted, the measure would give lifetime rental rights to low-income elderly occupants of converted apartment buildings. It would levy a tax of up to 4 percent on the value of condominium sales, and would grant tax benefits to landlords who sell or continue to rent to low-income tenants.

The bill is designed to stem a tide of conversions that is "devasting to the city" and threatens to wipe out more than one-quarter of its 150,000 rental housing units, John A. Wilson (D-Ward 2), its principal sponsor, told a news conference yesterday.

Wilson said 4,000 units have been converted, certificates have been granted for conversion of 14,000 others and an additional 20,000 units could qualify having existing law -- a total of nearly 40,000 of the city's 150,000 rental units.

He was joined at the conference by eight of the nine council sponsors -- representing a clear majority of the 13-member council needed for passage.

The proposed legislation came under immediate attack from spokesmen for the city's real estate industry, who called it confiscatory and promised an all-out fight, including a probable challange in court.

Condominium conversions in the District of Columbia have been subject to a near-total moratorium under emergency legislation first passed by the council last May and extended in July.

The council's second passage of the second emergency bill was ruled invalid lat month by a Superior Court judge, but the law remains in effect pending an appeal of the ruling to the D.C. Court of Appeals.

However, the second emergency law will expire automatically Nov. 25 unless the council chooses to ignore the current legal challenge and votes to extend it an additional 90 days at its meeting next Tuesday.

There was no word yesterday whether Mayor Marion Barry would support the far-reaching new measure introduced by the nine council members. Florence Tate, his press secretary, said the mayor had not read the bill and would have no immediate comment on it.

In the past, Barry has indicated support for conversion rules less restrictive than those now being proposed.

Among those who withheld support from the new bill were Council Chairman Arrington Dixon, Willie J. Hardy (D-Ward 7), chairman of the council's Housing and Economic Development Committee, and -- ironically -- Hilda Mason (Statehood-At Large), the sponsor of the original emergency condominium moratorium bill still in effect.

Mason has proposed two permanent bills of her own to regulate conversions, which Hardy has kept in a committee pigeonhole. Mason said yesterday day that one of those bills would produce greater safeguards for low-income tenants keeping their apartments than the bill supported by her nine colleagues.

Hardy would not discuss her plans for committee consideration of the new bill. Its sponsors said hearings are not likely to start until early next year. Passage involves a time-consuming procedure that normally takes several months, and requires a review by Congress that consumes at least an additional six weeks.

One novel feature proposed by the new bill is a vote by tenants of all apartment projects whose owners propose to convert to condominiums. At least 50 percent of the tenants would be required to vote in favor of conversion for the plans to go forward. The vote would be conducted by the tenants' association in each apartment building.

If a proposed conversion is rejected by the tenants, the developer would be barred for a year from seeking another vote.

An aide to Wilson said such tenant votes are required by law in New York City, New Jersey and in San Francisco and several other California cities, where they have been upheld by the courts. The precentages of required approvals vary from city to city, the aide said.

A study commission created by the D.C. council in conjunction with the existing moratorium recommended in September that 51 percent tenant approval be required for the conversion of low-rent units.

Sponsors of the new bill were candid yesterday in saying they hoped the measure would slow the trend to conversions.

Another key feature of the proposed new law would require that the individuals or corporations that convert an apartment building to condominiums must own property before applying for a permit.

A similar requirement recently was written into law by the Montgomery County Council as part of a package of bills designed to slow condo conversion. A 4 percent transfer tax also has been proposed for the county.

The measure would require condominium developers to grant lifetime rentals to persons 62 or older whose income falls below a figure set by a complex formula based upon family size and personal handicaps.

Real estate industry spokesmen voiced outrage at the bill. In New Orleans, where he is attending the National Association of Realtors convention, Washington condominium developer G. W. (Mike) Brenneman said the bill is "patently unconstitutional . . . a confiscation of property . . . and the real estate industry will fight it with all available means."

Brenneman is chairman of the national association's condominium committee.

John T. O'Neill, executive vice president of the Apartment and Office Building Association, echoed Brenneman's comment and said the group would seek to persuade the Court of Appeals to block the "pure subterfuge" of further emergency extension of the moratorium.