The National Cable Television Asssociation has signed a multimillion-dollar lease for all of the space in the seven-story building nearing completion at 1724 Massachusetts Ave. NW.

Currently located at 918 16th St. NW with 45 employes, NCTA plans to use four of the floors in the new building as a Washington headquarters to represent its 1,700 system members. They handle 65 percent of all cable subscribers and account for 45 percent of all cable systems in the United States.

"We plan to make the first floor a demonstration showcase for cable TV," said Kathyrn Creech, senior vice president of NCTA.

The 10-year lease involves two five-year options for renewal at a net rate of $15 a square foot, with NCTA handling management and service of all the space, three floors of which will be sublet in the range of $22 a square foot. Leggatt McCall & Werner represented NCTA in the negotiations and Arthur Rubloff & Co. represented the developer, SCI Corp., headed by a Greek investor with an office in London.

Angelos Dimitriou was the architect and Century Construction the builder of the structure expected to be ready for occupancy in February. The site is opposite the Brookings Institution in a special-purpose zoned area of downtown.

The NCTA plans to have 75 employes in its new headquarters by 1982.