The Essex, a 20-year- old, 167-unit rental apartment building on upper Connecticut Avenue will be converted to condominium ownership by the new owners, 4740 Associates.

G. V. (Mike) Brenneman, a principal in the purchasing group and a realtor specializing in marketing condominium and co-operative apartments, said that tenants in the fully occupied building have been notified of the purchase and told that they do not have to plan on vacating soon.

He pointed out that District law requires 120-day notification before eviction after a condominium conversion is undertaken but added that 30 to 50 per cent of the upper-income tenants are expected to buy the units. Many have lived there for years. Brenneman said major imporvements are planned for the building.

"If there are extenuating circumstances, such as illness or personal problems, we will make every effort to accommodate tenants," beyond the 120 day period, he said.

Althought Brenneman refused to divulge the price paid by the group of eight buyers, which includes investor Gerson Nordlinger, knowledgeable real estate professionals speculated that the total price was in excess of $4 million. That estimate was based on prices known to be asked or paid for other comparable apartment buildings in the range to $25,000 per unit.

Rents at The Essex range from about $200 to $560. It has a low rate of turnover in this generally transient Washington housing market.Brenneman said condominium prices would be highly competitive with other apartment building conversions here, where the range has been $56 to $60 a square foot.

Current tenants will get first choice to buy at below-market prices for the first 60 days of selling, he said.

An older, neighboring building at 4701 Connecticut, put on the condominium conversion market less than six months ago, is nearly sold, and some conversion along Connecticut Avenue from the Taft Bridge-Kalorama area to Chevy Chase.

In 1970, tenants at The Essex staged a short rent strike when rent increases averaging 18 per cent rent were instituted. Some tenants moved out as a result.