WASHINGTONThe Kennedy Center will build a major addition, expected to cost $100 million, to its iconic building along the Potomac River in order to accommodate needed rehearsal and education spaces as well as an outdoor performance area. The institution’s chairman, billionaire David M. Rubenstein, has pledged half the funding. The extension will be located just south of the building and connected by an underground walkway, according to a concept design created by architects Steven Holl and partner Chris McVoy of Steven Holl Associates, the firm that has been engaged for the project. Initial concept includes three connected pavilions that will house classrooms, rehearsal rooms, lecture space, multipurpose rooms and limited office space. As currently envisioned, one of the pavilions will float on the river and include an outdoor stage. A formal design will be created in the coming months, the center said. Ponying up $50 million after a previous donation of $25 million to the facility, Rubenstein becomes the center’s most generous donor. A new fundraising campaign will seek the remaining $50 million and an additional $25 million to support upcoming programming initiatives, according to the center. The project follows a previous and more elaborate expansion initiative that was announced in 2001 and nixed four years later after a $400 million funding measure was defeated in Congress. The current and more understated plans will encompass some 60,000 square feet, extra space that is needed in large part because of the Kennedy Center’s extensive education activities including the DeVos Institute for Arts Management. Additional benefits of expansion include a new rehearsal space sized to match the Kennedy Center’s Opera House stage. The building project is expected to take five years, three for design and approvals and two for construction.