Two Connecticut cities, Hartford and New Haven, had the lowest downtown office vacancy rates in the nation for the second quarter this year, according to a survey by the commercial realty firm of Cushman & Wakefield Inc.

Hartford had a vacancy rate of 7.7 percent, the third consecutive quarter that the Connecticut capital led the nation with the lowest vacancy rate. New Haven followed closely with 8.1 percent, a figure matched by Detroit.

The national average for the April-to-June period was 13.7 percent. The vacancy rate for the District of Columbia was calculated at 10.3 percent.

The worst vacancy rate in the nation was recorded in Austin, Tex., with 34.1 percent. Other cities with high rates included San Jose (29.8 percent), Denver (29.2 percent) and Miami (28.3 percent).

Rates in other major cities included Manhattan (9.6 percent), Boston (10.2 percent), Philadelphia (11.1 percent), Chicago (13 percent), Atlanta (14.4 percent), San Francisco (16.1 percent), Los Angeles (16.7 percent) and Houston (21.9 percent).

The 13.7 percent second-quarter vacancy rate for downtown office space is up from 13.5 percent. Similarly, the vacancy rate increased from 21.8 percent to 22.2 percent in suburban areas.

The lowest suburban vacancy rate was recorded in the Seattle area with 9 percent, while the Northern Virginia region ranked fifth with 13.3 percent. The worst suburban rates were in Boca Raton, Fla., with 38.3 percent, Austin (35.3 percent), Dallas (32 percent), Houston (31.4 percent) and Oakland (31 percent).

Arthur J. Mirante II, Cushman & Wakefield's president and chief executive officer, said that "despite the moderate uptick in the national office vacancy rates, major U.S. cities are returning to more balanced real estate markets. The severe oversupply of office space has begun to dissipate in many markets across the country, bringing demand more in line with new availabilities."

IN THE BUSINESS ... Centennial Development Corp. has broken ground for the Analytic Sciences Corp. headquarters building at the Reston Executive Center. The six-story building will be completed in June 1988. ... NewLife Homes Inc. has broken ground for the Stations at McDonogh, a 90-home development in the Baltimore area. ...Carydale Enterprises has announced plans to renovate the Dana Lyn Apartments in Laurel ... The Northern Virginia chapter of the American Institute of Architects has given Excellence in Architecture awards to four firms for five projects: James William Ritter Architect of Alexandria for additions and alterations on a private residence in Arlington, Davis & Carter of McLean for the Fair Lakes One office building, Kerns Group Architects of Washington for work at James Clothiers in Washington and for work on the south concourse at National Airport, and Alexandria Architects Partnership for work on the U.S. Postal facility at Quantico.

PERSONNEL FILE ... Stuart A. McFarland has been named chairman, president and chief executive officer of Skyline Holdings Corp. and chief operating officer of the workout committee of the court-approved reorganization of 332 EPIC limited partnerships, the Falls Church real estate syndication that entered bankruptcy in 1985.