Sun Co. Inc. of Philadelphia, one of the nation's major oil companies, is planning to redevelop almost a full city block near the Ballston Metrorail Station in Arlington into an 800,000-square-foot high-rise office and residential complex.

Arlington County land records indicate that Radnor Corp., a real estate development subsidiary of Sun Oil, has assembled most of the block bounded by Glebe Road, Vermont Street, Fairfax Drive, Taylor Street and Wilson Boulevard, including the Putt-Putt miniature golf course that long has been a Ballston landmark and the Glebe Court Apartments. Buchanan Co., a Rockville-based firm, will develop the project in partnership with Radnor.

The block, at the strategic intersection of I-66 and Glebe Road and adjacent to the Metrorail stop, has interested developers for the past five years, according to a spokesman for the Arlington Economic Development Division. Assemblage of a large piece of property by one developer -- in this case, Radnor -- usually means faster and denser development than if pieces had been purchased by many companies for separate projects.

Radnor has not purchased the three bank buildings on the block or a small office and retail building at the corner of Taylor and Wilson.

David Edwards, director of sales and marketing for Buchanan, said that the partnership would not try to purchase any of the bank buildings and that it would be several months before a specific development proposal is drafted.

"We've just started to look at the parcel seriously," Edwards said. "It will depend on how the market in Ballston keeps going, but if it continues to be as dynamic as we think it is now, we may be getting started before the fall of 1987."

According to the Arlington County Economic Development Division, Radnor/Buchanan could build as much as 400,000 square feet of commercial space and 400,000 square feet of high-rise residential space on the land it now owns. A spokesman for the county said it was unlikely the county would support a rezoning to increase the commercial part of the development and decrease the number of residential units.

"We will do what the county wants," Edwards said. "I don't know what our study will ultimately recommend for residential, but I would guess that it would not be town house but high-rise instead."

At the heart of the block are the Glebe Court Apartments, a 240-unit garden apartment complex built in the 1940s. According to Arlington land records, Radnor bought the complex from Glebe Court Partnership for $8.79 million last December. Glebe Court Partnership included William P. and Doris Ames, Alfred and Mary Farley Ames Lee, and Leon V. and Frances Ames Harrow.

Edwards said that Radnor will continue to rent the units for now but that it is unlikely that the company would consider renovating the garden apartments instead of replacing them with denser development.

"The apartments are sort of the hole-in-the-doughnut," Edwards said. "Without that land, it would be hard to do the kind of project we want."

Radnor bought the Putt-Putt miniature golf course from Wisconsin Builders Inc. for $4 million in May 1984 and several other small parcels along Taylor Steet during the past year from individual landowners for amounts ranging from $761,625 to $254,100. Deed transactions recorded by the county show that Radnor has spent $17.35 million assembling the land for the project.