An infusion of Canadian capital and approval of building plans by the Pennsylvania Avenue Development Corp. appear to have guaranteed a start within the next six months on the Lincoln Square office-retail complex at 10th Street and Pennsylvania Avenue NW.

Cadillac Fairview, a Canadian firm with an increasingly large interest in U.S. realty projects, recently became the major investor in development plans for the downtown block bounded by 10th, 11th and E streets. It is north of the so-called "ceremonial avenue" that gradually is being renewed under the aegis of its own development commission.

Cadillac Fairview purchased the Lincoln Square site interests held by Richard Cohen, John Mason and Stuart Bernstein, said developer David Evans. Contracting executive A. James Clark remains as a minor partner in the Lincoln Square development. Additionally, the Arthur Morrissette family has control of one existing building on 10th Street that will be redeveloped in the block.

The complex will replace a block of low-rise, turn-of-the-century commercial buildings, which the development commission describes as largely "deteriorated."

The 745,000-square-foot Lincoln Square complex is expected to cost more than $50 million, and will incorporate five existing buildings and/or facades."

The complex "will resemble an urban skyline and adjust to existing buildings," said architect George Hartman of Hartman & Cox, the project's architectural firm. The firm of Smith, Segreti & Tepper is working with Hartman-Cox in an associate capacity on the project.

The buildings in the complex will range in height from 40 to 160 feet -- unusual in downtown Washington, where a maximum height of 12 stories is almost standard. (Buildings on Pennsylvania Avenue NW may go up to a maximum of 160 feet in height if the buildings in question are set back 50 feet from the street.) The complex will be built of brick, limestone and precast concrete.

Several blocks to the north of the two-acre Lincoln Square site, developer John Akridge has acquired a 99-year ground lease on the 66-year-old Homer Building at 601 13th St. NW.

For a partnership that includes Penn Mutual Life Insurance Co., Akridge obtained the lease from S. Kann Sons Co. for an undisclosed sum after three years of negotiations. The four-story white terra cotta building, which sits on a 24,000-square-foot site, is believed to be worth nearly $10 milllion in today's downtown real estate market.

Akridge, whose firm recently completed two new buldings near 15th and L streets NW and has another spot for a new building on the site formerly occupied by the YWCA at 17th and K streets NW, said that the lease specifies that the building be turned over to the partnership in 1985. He said he will consider development alternatives in the interim.

The building, which sits over an entrance to Metro Center at 13th and G streets NW, has housed Strayer's Business College, the Galt jewelry store and a Hahn's shoe store.

In another new development in the downtown area, the Oliver T. Carr Co. -- recognized now as the prime local developer of downtown properties -- recently acquired a site at 854 M St. NW for $2 million. A Carr executive said that no immediate development is planned for the site near the new campus of the University of the District of Columbia, Mount Vernon Square and the new convention center. However, Carr now is acquiring properties in the area for under $200 a square foot with an eye to future construction.

The Carr firm has major building projects under way at 15th and G, at 18th and K and at 212 K St. NW, in addition to a new downtown condominium apartment building and plans for additional development in the West End.

In the area northwest of Capitol Hill, the Georgetown University Law Center is to expand to most of the block bounded by 2nd and G streets NW and New Jersey and Massachusetts avenues. A university spokesman said that the law library and administrative offices need more space and that a large building will be started there within a year.

Meanwhile, the Mark Winkler Co. has sold the 41-year-old Commonwealth Building at 1625 K St. NW for $13 billion to a group of out-of-town investors. The 12-story building on a 8,100-square-foot site was developed by Harold Winkler and his late brother Mark. The Winkler company recently has been active in developing apartments, campus-like office buildings in the Hamlet area at Seminary Road and Beauregard Street in Alexandria. The firm recently obtained approval to build a 30-story Radisson hotel on part of the large Hamlet site.

Union Labor Life Insurance Co., now based in New York City, also has exclusive rights to purchase a redevelopment building site at 2nd Street and Massachusetts Avenue NW, near Union Station. The insurance company reportedly is studying plans to put a headquarters building on the site.

Finally, the National Association of Realtors moved earlier this week to the Wyatt Building at 777 13th St. NW.The Chicago-based trade association bought the building 18 months ago to accomodate its expanding Washington staff, which outgrew the small building (formerly occupied by B.F. Saul Co.) that it had owned and used for five years at 1025 15th St. NW.