A one-acre downtown site for a new office building has been purchased by the International Bank for Reconstruction and Development for about $10.4 million.

A spokesman for the World Bank confirmed the purchase of a tract at 19th and I Streets NW from 1850 I Street Holdings (a group headed by developer Ben Jacobs) as the site for a 12-story office building that will be started soon. The structure, which will have more than 40,000 square feet of usable space, is expected to be ready for occupancy by 1,500 World Bank employees early in 1980. There will be parking on four below-grade levels.

The site, opposite the former Doctors Hospital properties being redeveloped as the International Square complex, currently is the site of a parking garage and some small buildings. It is understood that the Jacobs group was well along in the planning of a new structure on the 44.130 square-foot site before the sale was made for $235 a square foot.

The World Bank is an international development organization based at 1818 H St. NW. Its present facilities include 900,000 square feet of space in several buildings.

The bank has been negotiating with neighboring George Washington University to develop a new building in the block surrounded by 18th and 19th and F and G Streets NW. That plan has been delayed by attacks from several groups because some historic buildings in the area have been torn down.

The transaction for the property at 1850 I Street is regarded in real estate circles as an indication of the increasing price being paid for prime office building sites and also because it marks the first move of the World Bank to the north side of Pennsylvania Avenue.