Rozansky & Kay Construction Co. disclosed plans yesterday to develop a $50 million, 12-story office complex at 11th, 12th and E streets NW, a prime downtown commercial site one block north of the Pennsylvania Avenue Development corridor.

Construction of the building, which will begin next year, is planned as the second phase of Rozansky & Kay's development of roughly 40 percent of the block that is bounded also by F Street NW.

The first phase of development on the site is scheduled to start next fall when the company begins refurbishing an eight-story building that houses executive offices and the main branch of Perpetual Federal Savings and Loan Association. Rozansky & Kay recently purchased the building from Perpetual American for more than $14 million.

The Perpetual American building is adjacent to a parking lot on which Rozansky & Kay will build a 165,000-square-foot building.

Perpetual American has agreed to lease space on the first floor of the refurbished building, where it will continue to operate its main District branch. The S&L will also lease space on the mezzanine level but hasn't yet decided whether to locate its executive offices there, said Perpetual American President William Sinclair.

It's possible, he said, that the executive offices will be relocated to Perpetual American's Capitol Hill branch at Pennsylvania Avenue and Third Street SE.

Sinclair said the decision to sell the building at 11th and E streets was prompted by the need for additional space. Perpetual American built the marble-facade building in the early '50s but the company's growth over the years forced it to house employes at several locations throughout the area.

Earlier this year Perpetual American began moving most of its operations division to a new building in Alexandria.

The remaining floors in the existing building and all but the first floor of the structure planned by Rozansky & Kay will be developed for general office use, said Alan I. Kay, cofounder and partner in the Bethesda-based development company.

The new building will contain an atrium and first-floor shopping arcade that will extend from 11th Street to 12th Street.

Kay said the new complex, which will have a marble or granite exterior, will be designed to blend with the architecture of surrounding structures, several of which were completed within the past two years.

The architectural firm of John S. Samperton & Associates has been retained to design the Kay building, which is scheduled for occupancy in 1985.