Joseph A. Califano Jr., Secretary of Health, Education and Welfare, signed a regulation yesterday to a four-year-old law forbidding discrimination against the handicapped. He stated that the regulation would "work fundamental changes in many facets of American life" and "opens a new era of civil rights in America."

The controversial regulation would affect thousands of schools, colleges, hospitals, day-care centers, health clinics and other institutions receiving HEW funds.

The regulation, which will go into effect in 30 days, would, in general provide that:

Handicapped children can no longer be segregated in public schools but will have to be educated with non-handicapped in regular classrooms to the "maximum extent possible." At present, experts estimate several hundred thousand handicapped children are educated in special separate programs.

Every handicapped child -- no matter the nature or severity of the handicap -- will be entitled to a free public education.

HEW general counsel Peter Libassi estimates that at present, 1 million handicapped children are being denied public education. With this rule there must be prompt, full compliance by local school districts and "under no circumstances . . . later than Sept. 1, 1978." In those "unsual cases" where a child must be placed in a special residential school, public authorities --not the parents, as is now often the case -- will be financially responsible for tuition, room and board.

Colleges and universities cannot house the handicapped in segregated buildings or wings nor cluster them in segregated classroom buildings. By Aug. 1, all programs or activities must be made accessible to the handicapped by such means as reassigning classes, hiring aides to interpret for the deaf, buying books in Braille. If such methods do not work, structural changes -- installing elevators or ramps, for example -- must be completed in three years. No exceptions will be allowed.

Any new facility built after June 1 must be barrier-free and readily usable by the handicapped.

Employers cannot refuse to hire the handicapped if the handicap does not impair the ability of the applicant to do the job and if "reasonable accommodations" can be made to the handicap.

The first major federal civil rights act for the handicapped, passed by Congress in 1973, has been controversial from the beginning because of its broad and unequivocal wording and because of the wide range of services it would affect and the wide range of handicaps it covers. The previous administration spent 3 1/2 years drafting a regulation, but then-HEW Secretary David Mathews refused to sign it, sending it back to Congress for a further definition of intent.

Califano took four months to re-examine the regulation and was beseiged by leaders of handicapped groups who demonstrated and sat-in at HEW offices here and across the country demanding his signature. They proclaimed a victory yesterday on major points they had fought to keep in the regulation.

One of the most controversial drug addicts and alcoholics. Califano was considering deleting them from the regulation until an opinion by Attorney General Griffin B. Bell stated that alcoholics and drug addicts were covered by Congress' definition of handicapped individuals.

However, Califano emphasized handicapped group is that the regulation applies "only to discrimination against qualified handicapped persons solely be reason of their handicap." In other words, an institution may refuse services or withhold employment to an alcoholic or drug addict --just as it would to any person -- "on the basis of past personnel records, absenteeism, disruptive, abusive or dangerous behavior and unsatisfactory work performance."

A "conservative" estimate of those classified as handicapped is 35 million, about one in six Americans, said Libassi. This does not include an additional 11 million alcoholics and drug abusers.

Institutions the regulation will affect have been warning for many months that its cost could prove prohibitive, and that it could also prove disruptive in a number of ways. HEW estimates the cost at $2.4 billion, most of which would be required in public school systems for evaluation and special services to handicapped children.

The theoretical penalty for failure to comply within the regulation is loss of federal aid.

A "handicapped individual" is defined as "any person who has a physical or mental impairment which substantially limits one or more of such person's major life activity" -- such as being blind, deaf, crippled, mentally retarded, mentally ill. It also includes anyone who has a record of such an impairment or is "regarded" as having such an impairment Thus, a person who had cancer and is still "perceived" as having an impairment could not be refused a job solely on that record of illness.

School officials in the Washington area felt the impact of the new regulation might be limited here because most school jurisdictions already operate extensive special education programs.

"We don't anticipate having to make any changes in our special education program because of the regulation," said Paul Masem, an associate superintendent in Montgomery County. "We had assumed all along that we could implement the regulation."

George McKinney, director of federal programs in Prince Georges County, said his county also is probably already providing an education program that meets the requirements of the regulation. But McKinney said officials will now take a closer look to make sure.

Fairfax County, which serves 11,400 disabled students in an array of education programs for the handicapped, is expected to spend an additional $500,000 a year on special education as a result of the regulation, to Dr. John Markwood, contract services specialist for the county.

In Washington, which is under a special court order by U.S. District Judge Joseph C. Waddy to educate its 14,000 handicapped students, few changes are expected to result from Califano's actions, according to Hilda Mason, a City Council member and former member of the D.C. School Board.