Downtown office leasing continues strong in 18 major metropolitan areas, particularly in Washington.

Preleasing of new space at record rates is becoming commonplace, and a shortage of large spaces is likely in 1981, according to a report from the Office Network, of which the Braedon Companies here is a founding member.

J. Fernando Barrueta, senior vice president of Braedon, commented that the space in new downtown buildings is being preleased heavily at rates that include rising escalator cost factors tied to the consumer price index. However, he also pointed out that there is mounting evidence of a surplus of new office space in several Northern Virginia locations, including the fast-growing Tysons Corner area.

The Office Network report showed only a 0.3 percent vacancy rate of downtown office space here in contrast to a 5 percent vacancy factor for space outside the central business district. In Baltimore, the downtown vacancy rate is 11.6 percent and 8.2 percent for outlying area space. Boston, Los Angeles and San Francisco have low vacancy rates, whereas the percentage of unleased space is highest (17 percent) in downtown Atlanta.

The nation's most expensive new office space is being leased, not suprisingly, in New York City, where the rate now is passing $50 a square foot. In Washington, new space now is being committed at rates exceeding $20 a square foot, but the average rate here is still below $16. New office spaces generally are being leased at rates 23 percent higher than rates for in-place space.

Elsewhere on the building and leasing scene here:

Walker & Dunlop has signed First American Title Insurance Co. as a tenant for the top floor of the Providence Building in the City of Fairfax. tThe space will be used for the mid-Atlantic regional headquarters of FATI. t

Shannon & Luchs has been named exclusive leasing agent for Courthouse Plaza II, a development of 100,000 square feet being built by Poretsky & Starr in the City of Fairfax. Completion is scheduled this spring.

Braedon announced that Werner & Associates, a computer-service organization, has leased 12,600 square feet at 1600 Wilson Blvd. in Rosslyn. oThe Werner firm plans to relocate its headquarters office from the District. tBarnes, Morris & Pardoe represented Werner in the negotiations.

Weaver Bros. Inc. announced the sale of the former Food Fair warehouse at 2060 W. Virginia Ave. NE. Minnix Associates sold the 45,000-square-foot building on the New York Avenue corridor for $420,000 to a partnership group. Goodwill Industries is one of the tenants.

Fred Ezra, formerly a leasing executive with the Julien J. Studley firm here, has opened his own commercial real estate brokerage and leasing office.

The Storage Place, located in a rejuvenated older building at 175 R St. NE (also in the New York Avenue corridor) now offers self-storage container space at rates ranging from $8.50 a month for a 4-by-4 foot container to $109.50 for a 10-by-20-foot container. Lucien Sichel is one of the four partners who own the large storage building, built 51 years ago to house a bakery.