Half a billion dollars' worth of construction is scheduled to start on Pennsylvania Avenue in the next two years, as redevelopment of "A merica's main street" enters its most active phase.

Three new hotels, dozens of shops and thousands of offices will be built in a row of new buildings stretching along the north side of the avenue from Seventh to 15th streets.

Along with the previously announced plans for reopening the Willard Hotel, renovating the National Press Building and raising a massive Marriott Hotel, developers are preparing half a dozen other projects.

They range from the $8 million renovation of the Apex Liquors building and the adjacent structure, where photographer Mathew Brady kept his studio, to a pair of $100 million-plus megaplexes.

On the block between 10th and 11th streets, developers David W. Evans and A. James Clark are planning the biggest Pennsylvania Avenue project so far, a $150 million, 15-story structure.

Evans and Clark are planning a partnership with the giant Cadillac-Fairview Corp. of Toronto on the project, which will be known as Lincoln Square, 1001 Pennsylvania Ave.

The Pennsylvania Avenue Development Corp. has already approved the design of the project, which won an award from the local chapter of the American Institute of Architects and was endorsed by Don't Tear It Down, the building preservation group.

Designed by Washington architects Hartman-Cox, the building will incorporate five existing buildings into a complex with a big interior atrium, 650,000 square feet of office space and 60,000 square feet of shops.

Almost as large is the project to be built by Westminster Investing, an affiliate of the B. F. Saul companies. A 350-room "European style" hotel, 250 apartments and a large office building will be included in the $100 million-complex.

Plans are being prepared by architect Leo Daly of Washington to include in the structure what was once the terminal of the Atlantic Coast Line Railroad at the corner of Pennsylvania Avenue and 7th Street. The site includes part of C Street, which PADC plans to close. Saul interests own more than half the property and next month will ask the PADC to use its condemnation power to acquire the remainder.

The giant Saul project is next door to the Apex and Brady buildings, which Hartman-Cox has encorporated into a preservation and enlargement project.Developers are U.S. Land Resources Inc. and three Washington investors, attorney Gary Tabak and Bruce Berger and architect Andrew Kinzler.

Work began a few days ago on a $115 million hotel and office buiding developed jointly by Marriott Corp. and Quadrangle Development, at 14th Street. Quadrangle is preparing to open a $17 million office building at the other end of that block, 13th and Pennsylvania.

On the back of that block, the National press Club will seek PADC approval next week for a $28 million renovation of the National Press Building.

Across 14th Street, demolition work is under way at the Willard Hotel, although developer Stuart Golding is having trouble financing that project.

Later this spring tenants will begin moving into 1201 pennsylvania Ave., between 12th and 13th streets, a $38 million investment by Cabot, Cabot & Forbes of Boston.