Office leasing in Fairfax County has increased more than 30 percent in the first seven months 1982 compared with the preceding six-month period, according to the county's economic development authority.

Locally based companies continue to absorb the majority of office space on the market, with recent leases in the county going to McDonnell Douglas, MCI, Tymshare/Tymnet, Olivetti, Titan Systems, Vinnell and Rolm-Mid Atlantic. These firms, which are concerned with telecommunications, computer development and professional services, are typical of the companies that previously located in Fairfax, the county office said.

From January to July of this year, the market absorbed about 166,000 square feet of space per month.

In the last 3 1/2 years, office space leasing in Fairfax County has taken up 5.8 million square feet. The largest amount--about 200,000 square feet--was leased between November 1979 and April 1980. Between April 1980 and January 1982, the market absorbed about 166,000 square feet per month.

The county estimates that about 1.5 million square feet of completed space is immediately available there, in addition to 14 office buildings currently under construction. The vacancy rate is estimated at about 5 percent.

Recent developments in commercial leasing in the Washington metropolitan area:

* Martin Marietta has leased an additional 13,000 square feet of space in Building III of the Capital Office Park in Greenbelt. The company already occupies about half of the park's Building II. The aggregate rental of the transaction is about $1 million. The park is owned by Eleventh Springhill Lake Associates and managed by Community Realty Co. Inc.

* MIW Investors of Washington has sold a 24.7-acre commercial parcel in Margate, Fla., to the Hospital Corp. of America, which intends to build a hospital on the site. The purchase price was about $2.35 million. MIW realized a gain of about $445,000. It will retain an adjacent 6.4-acre parcel for future development.

* The 13,000-square-foot space in Canal Square occupied by the Port of Georgetown restaurant has been leased by Ray Giovanonni, owner of the Fish Market and Il Porto restaurant chain. Giovanonni will open Fish Market and Il Porto outlets like those in Old Town Alexandria, Richmond and Wilmington in the Georgetown space. The aggregate value of the long-term lease is estimated at about $2.5 million.

* The Sanz School Inc., which offers English and foreign language classes, has leased 3,588 square feet at 11 Dupont Circle NW.

* The Tangent Corp., a firm that provides secretarial services, has leased 1,979 square feet of space at 1901 L St. NW.

* Aurora Data Corp., a word processing and business computer sales company, has leased 1,387 square feet at 1600 Wilson Blvd. in Arlington.

* The law firm of Barnett & Alagia has signed a 10-year lease for about 22,000 square feet at 1000 Thomas Jefferson St. NW. The firm, which was previously located at 1627 K St. NW, also purchased a residential condominium at Wadsworth House at 1045 31st St. NW.

* Global Research has sublet about 4,500 square feet at 2828 Pennsylvania Ave. NW from Syscon Corp. The total lease value is over $270,000.

* Hekimian Laboratories Inc. has leased 25,000 square feet of space at 8435 Helgerman Court in Gaithersburg. The lessor is Halcyon Associates.

* Furstenberg Beer Importers Inc. has leased 2,044 square feet at 9055 Comprint Court, also in Gaithersburg. The lessor is Merit Associates.

* McDonnell Douglas Corp. has leased over 12,500 square feet at Flint Office Park in Fairfax. The aggregate rental of the long-term lease is about $1 million.

* The American Society for Public Administration has leased over 5,000 square feet of space at 1120 G St. NW. The aggregate rental of the long-term lease is about $1 million.