More than 14 months after the Kennedy Library Corporation decided not to build its JFK memorial library on the MBTA Yard site across from Eliot House, the state has begun a search for a private company to develop 4.21 acres of the 12-acre tract.

An advisory committee charged with recommending a developer to the state yesterday placed advertisements in the New York Times, The Washington Post and The Boston Globe, offering the land for commercial development.

Many Uses

Under a comprehensive development plan for Harvard Square, the MBTA Yard site is appropriate for a combination of residential and commercial uses including office space, retail stores, hotels, restaurants or off-street parking.

The land on the site was appraised several years ago at a value of $16 per square foot, but a new appraisal setting the exact sale price is expected to be released within the next two weeks.

Recent estimates have set the value of the 4.21-acre parcel as high as $20 million.

The rest of the MBTA Yard site is divided between a proposed 5.5-acre Kennedy Memorial Park to be located next to Memorial Drive and a 2.91-acre site for a building to house Harvard's Kennedy School of Government, the Kennedy Institute of Politics, and other related academic departments.

Demolition has begun on Harvard's site, and workers will begin foundation work for the building in April, Donald C. Moulton, assistant vice president for government and community affairs, said yesterday.

Moulton said he expects the building to be ready foroccupancy in the fall of 1978.

The MBTA will not vacate the site, however, until some time later. The temporary Harvard Square subway station will be built on the edge of the commercial development site while the Red Line is being extended to Arlington.

One of the MBTA's electrical transformers and some of its railroad sidings will remain on the Harvard site for several years, Moulton said yesterday.

State Rep. Thomas H. D. Mahoney of Cambridge, the chairman of the Re-Use Committee that the state legislature established for the site last year, said yesterday that the committee is "interested in finding the best and most imaginative developer" for the commercial parcel.

The Comprehensive Development Plan for Harvard Square has suggested a height limitation of about 80 feet on new construction. Moulton said the Harvard building would be in the 40-45 foot range, about the same as neighboring Eliot and Kirkland Houses.

Mahoney's committee set June 1 as the deadline for submission of development proposals, and will hold public hearings this fall on the plans of the five or six finalists.

The state originally bought the entire site from the MBTA and donated it to the federal government as the proposed home for the John F. Kennedy Memorial Library.

Community Pressure

After the Library Corporation decided to put the library at Columbia Point in Dorchester because community groups in Cambridge objected to building the library here, the land was reverted to the state for disposition.

The other members of Mahoney's committee are Lt. Gov. Thomas P. O'Neill III, Cambridge City Manager James L. Sullivan, Frank Keefe, the state planning director, and Martha Lawrence, a Cambridge resident.

The 4.21-acre parcel will be the only part of the site of generate tax revenue for the city.

The Metropolitan District Commission, the agency that will build and maintain the Kennedy Park, recently solicited bids for the construction, but has yet to select a contractor.