The owners of the Detroit Express soccer team are negotiating with a group of Washington investors with an eye toward moving the franchise to RFK Stadium for the 1981 season, according to a team owner.

Gerritt B. Lemmen, one of eight partners in the team's ownership, said yesterday that Duncan Hill, another partner, has been talking to a Washington group that is interested in buying all or part of the franchise.

RFK Stadium currently is vacant in the spring and summer. The Diplomats, the North American Soccer League franchise here the last seven years, folded last November and the owner, Madison Square Garden Corp., voluntarily terminated the team.

A move or a sale would have to be approved by two-thirds of the Detroit partners. Lemmen declined to name any members of the Washington group, but former Diplomat president Steve Danzansky is not involved in the negotiations, Hill said last night.

"I would expect we'll know sometime next week whether this deal will go through or not," Lemmen said.

Hill, who also is the team's general manager, said last night that the Washington group comprises local businessmen, never before involved in soccer. "But they were very upset when the Diplomats folded," he said.

Hill also said that if the franchise moved here he would definitely want the former Diplomat coach, Gordon Bradley, involved in some capacity. "I think having Gordon would be essential," he said.

The Express came into the North American Soccer League in 1978 as part of a disastrous expansion from 18 to 24 teams. Although the club won its division with a 20-10 record in its first year, the Express has never been a success at the gate. Detroit finished last season out of the playoffs with a 14-18 record while averaging about 10,000 fans per home game.

"I don't see that changing very much in this area (Detroit) in the next three to five years," Lemmen said. "This part of the country is going through a very bad time economically, and I think it's going to be tough for soccer to make a go of it here the next few years.

"Washington is clearly an excellent soccer town. They averaged over 19,000 per game last year. I know a lot of that was (Johan) Cruyff but I think we could average 15,000 there and I would be more than satisfied with that."

Lemmen added, "The odds are still against it (the move) because nothing's concrete. We'll know a lot more after Duncan gets back this weekend. But it's certainly a possibility. I'm sure the league would cooperate."