The Reagan adminiration's proposals to put "real teeth" into the 15-year-old Fair Housing Act were disparaged yesterday by Sen. Edward M. Kennedy (D-Mass.) as "plainly inadequate . . . , a half-hearted approach."

Kennedy's statement echoed the reaction of the Leadership Conference on Civil Rights, a coalition of 165 national civil rights groups. In a letter yesterday to members of the House of Representatives urging support for a separate proposal, the conference charged that the president's proposal would produce "costly, lengthy and inefficient" lawsuits to be handled by an already overburdened court system.

Now the Justice Department can sue only in cases involving a pattern or practice of systematic discrimination, not in individual cases. The administration's bill would allow Housing and Urban Development Secretary Samuel R. Pierce Jr. to send allegations of discrimination against an individual to the Justice Department with recommendations that lawsuits be filed.

The civil rights coalition's letter urged members of Congress to support a measure by Reps. Don Edwards (D-Calif.) and Hamilton Fish Jr. (R-N.Y.)--virtually identical to a bill introduced in the Senate by Kennedy, Sen. Charles McC. Mathias Jr. (R-Md.), and 39 others--that would create administrative law judges at HUD to handle complaints.