A Houston firm and two Fairfax builders are planning to construct more than 600 housing units -- including garden apartments, condominiums and town houses -- within the 620-acre Fair Lakes mixed-use development in the Route 50/Interstate 66 corridor of western Fairfax.

Lan Bentsen Interests, a Houston-based company with apartment projects in six states, announced its intention this week to build about 298 rental apartments at West Ox Road and Fair Lakes Parkway.

Meanwhile, Scarborough Corp., a Fairfax-based company, said it plans to build about 250 "move-up condominiums" with starting "prices at least at $100,000" north of Monument Drive along the section of the Springfield Bypass that already has been built by developers of Fair Lakes.

Bob Batal, president of Batal Builders of Fairfax, said his company will erect between 110 and 115 New England-style town houses at Route 50 and the bypass.

The construction of these residential units would fulfill 46 percent of the commitment to build housing made by Hazel-Peterson Cos. to Fairfax County when the Fair Lakes project was approved, a Hazel-Peterson spokesman said.

Fairfax Chamber of Commerce leaders and building industry officials welcomed the announcements of all three projects, but were especially pleased with the Texas company's plan to build rental housing, a rare commodity in today's real estate marketplace throughout Northern Virginia.

Chamber spokesman Clark Massey said that in Fairfax County "as of Jan. 1, 1984, there was only 62,000 acres of vacant or under-utilized land. Only 2 percent of that land, under the county's land-use plan, was capable of being developed at more than eight units per acre."

Sam Finz, chief executive officer of the Northern Virginia Builders Association, said, "Any kind of rental housing is a major plus. The lack of rental housing, apartments or houses is one of the bigger problems we face in Northern Virginia."

He praised the apartment developer's statement that rental rates would be in the $600-per-month range. "That is really good, right in line with what is needed," Finz said.

Lan Bentsen, president of the Texas firm that bears his name, said his company is "very excited about the Virginia market." This will be its first Virginia development, but he said his company is looking at other sites in Northern Virginia.

He said market studies showed there is "a good and sound demand" for apartments in Fairfax. Bentsen said he plans to build apartments "aimed at those making $30,000 to $40,000 a year." The garden-style apartments will be in two-story and three-story buildings and will offer six different floor plans.

"The project will be very low density, relative to Houston, with 20 units per acre," he said.

Scott Beatty, vice president and division manager for Scarborough, said his company's development will include two- and three-bedroom "move-up condominiums" starting in the $100,000 price range, geared to those working in the "high-tech marketplace" that will comprise major portions of the commercial and office development within Fair Lakes.

Batal said he plans to build two- to four-bedroom town houses in the $150,000-to-$165,000 price range.

"All will have a single-car garage and some will have double garages," Batal said. In recent months, real estate agents and builders throughout the Washington area have said that town-house buyers are increasingly demanding garages with move-up town-house units, a feature lacking in the majority of developments.

Bentsen, Beatty and Batal all said the location of the Fair Lakes development was a key to their decision to build residential units there. The mixed-use project is between I-66 and Route 50, west of West Ox Road. Hazel-Peterson Cos. already has built portions of the future Springfield Bypass that will run within the project and is now paying for construction of a new interchange at the bypass and I-66.

Bentsen said his firm will join with Hazel-Peterson to build the apartment units. Scarborough and Batal have purchased the land where they will build their projects.

Both Scarborough and Batal said they plan to start selling their homes next summer. Final site plans, which will determine the exact number of units in each of the three projects, still must be approved by Fairfax officials.

Fair Lakes, when it is finished, will include more than 5 million square feet of office, retail and hotel space and 1,300 residential units.

TRW's Defense Systems Group has purchased 120 acres at Fair Lakes, where it plans to build a campus-style office center.

Mohasco Corp., America's largest interior furnishings company, also plans to move its corporate headquarters to Fair Lakes from Amsterdam, N.Y. Construction also is under way on a new corporate headquarters building for the Hazel-Peterson Cos. that overlooks a lake within the development.