A short-term land deal cut by the regional high school committee Monday night has flung open the starting gates on an ambitious $6-million plan to build an addition to Martha's Vineyard Community Services (MVCS), doubling the space at the social services agency.

After more than a year of negotiations, the school committee finally agreed to loosen its grip on a one-acre sliver of school-owned land next to Community Services. The parcel will be leased to the agency for three years, allowing it to continue operation in modular buildings during construction.

For school committee members, the stumbling block was removed once they knew the land use would be temporary. School leaders have refused to make any permanent decisions about land until they can hire their own planning consultant to study the 72 acres owned by the school on both sides of Edgartown-Vineyard Haven Road.

Specific terms of the agreement have not been decided, but officials on both sides of the table viewed Monday night's action as a green light to proceed and as a sign of more collaboration between the high school and Community Services.

Ned Robinson-Lynch, the director of Community Services, said that with the planned expansion, space dedicated to high school students will grow from 700 square feet to 1,500 square feet. "In the last couple years we've worked closely with [school superintendent] Kriner Cash and [high school principal] Peg Regan, and we are excited to continue to partner to benefit the Island youth," said Mr. Robinson-Lynch. "These are our kids, and we want to provide services for them."

And beyond the needs of teenagers, demands on the agency have increased. In the last 12 years, programs and personnel have doubled, said Mr. Robinson-Lynch. The caseload for substance abuse issues alone has tripled in the last four years. Day care, nursing, mental health and drug counseling and special support for women are among the services offered there.

But the building has not kept pace. Space is the biggest problem, but the physical complex of the three buildings has also deteriorated. Some parts of the structure are prone to flooding, and other parts are poorly heated, said Mr. Robinson-Lynch. With a state grant, Community Services hired John Abrams of South Mountain Company to draft a feasibility study and propose a project that would transform the complex from one-and-a-half to two-and-a-half stories tall.

"This would put Community Services in good shape for the foreseeable future," said Mr. Abrams, who attended the school committee meeting.

Along with news of the land deal, Mr. Robinson-Lynch began to outline a capital campaign to help raise the nearly $6 million he expects the expansion to cost. He hopes that through a combination of fundraising, grants and loans, that goal will be reached.

Groundbreaking on this project is at least a full year away, and Mr. Abrams is working on a detailed timeline for requesting proposals, screening architects, designing plans and hiring a contractor.

If Community Services had not reached an agreement with the school committee, Mr. Robinson-Lynch said, his board would have been forced to relocate elsewhere on the Island. But this week, school committee chairman Ralph Friedman emphasized the need for Community Services to remain close to the high school.

"The high school really needs the expertise and planning of Community Services," he said.

In a memo passed out to school committee members, current services offered to high school students were spelled out. Support is offered to students struggling with substance abuse or teen dating violence and to teenage mothers and victims of sexual harassment. Mr. Robinson-Lynch said his agency wants to take part in master planning efforts at the high school to build on those collaborative services.

As part of this week's agreement, the one-acre parcel of land just to the west of Community Services will be cleared of trees and readied for the temporary modular buildings. At the end of the construction project, the buildings will be removed and the site cleaned.

Community Services is currently sited on three acres leased from the high school.

The compromise struck by the school committee this week opened the way for Community Service to move forward, but the school's land use committee must still deal with requests from proponents of a skateboard park and aquatic center, who both want a piece of high school land.

But school leaders are reluctant to make deals that are permanent. This week, members of the school board repeated their calls for a plan that can give an overview of school real estate and forecast the growth of student population. The land could become a site for a regional middle school and possibly a sewer plant.