William L. Berry & Co. Inc. has purchased 262 acres of prime residentially zoned land in the River Road area of Potomac from the Sisters of Mercy of the Union of the United States of America.

Berry, a Maryland-based residential developer, has built major projects in suburban Virginia and Maryland. Neither the developer nor a spokesman for the religious order would reveal the sales price of the Potomac land.

The sale by the Sisters of Mercy completes their disposal of 1,000 acres along the River Road corridor that formerly housed the religious order and a major school that first occupied what was known as the Kendall estate in 1930.

Over the years, portions of the original tract have been sold. In 1980, the Sisters sold 346 acres to the U.S. Postal Service for construction of a management training facility for $6.8 million.

The latest sale by the Sisters represents the last of their holdings in the old Kendall estate, according to Peter Shannon, an attorney representing the Sisters of Mercy.

"This is the last large piece of undeveloped land in close-in Potomac which is residentially zoned," said William. L. Berry, president of the development company.

An agent for Shannon & Luchs' Potomac office described the land as "a very prime piece of ground." It is adjacent to the prestigious Falconhurst development, where prices for homes now top $1 million. The Berry site is close to the Bethesda/Potomac line in the Bradley Boulevard area, a fact that greatly enhances the property's value, one agent said.

Berry called the chance to develop the site a "unique opportunity to offer people a chance to live in Potomac," considered one of the most prestigious suburban addresses in metropolitan Washington.

Berry is planning to combine custom homes with some of his own models that he expects to sell in the $500,000 to $800,000 range. A special section also will include homes in the $300,000 price range, Berry hopes.

The major access to the new homes will be via Kentsdale Drive, which he said will be opened through the development.

Berry plans to develop 70 two-acre lots and 19 half-acre lots himself. Another section of 26 two-acre lots will be set aside for sale to custom-home builders or to individuals wanting a site to "build the house of their dreams," Berry said.

Berry said the project already has been zoned and the site plans approved.

The section that will include the 19 half-acre lots will have a separate entrance. It will be in this section that Berry said he will try to keep the prices in the $300,000 range and try to repeat what he has done in the McLean area of Virginia, where he has built homes at slightly below market prices.

Several years ago, Berry began construction of McLean Estates on Lewinsville Road in the McLean area with advertisements that beckoned purchasers to buy the homes he was building at below market rates. His models were selling starting at around $200,000. Houses nearby were topping $400,000 to $500,000.

The Sisters of Mercy order is primarily dedicated to the causes of the poor, needy and the homeless, and the order has been extensively involved in education for the handicapped.

In 1980, the order moved to less elaborate quarters in Silver Spring. It has sold off the original 18-room mansion, which is now being used as a monastery. A subdivision known as McAuley Park also was built on land sold by the Sisters.

For more than 21 years, the Sisters operated the St. Maurice School on the property. That school, which closed four years ago, was considered a model for other schools specializing in the needs of emotionally handicapped children.

The Postal Service's plans for its training center on that slice of the original estate created a furor in the Potomac area that finally ended in a U.S District Court decision allowing the Postal Service to build the facility. The McAuley Park Citizens Association had claimed construction of the facility would have severely damaged the environmental quality of their neighborhood.