It's a scene repeated in more and more households as '80s economics take their toll: adult children strike out on their own, then return to the nest because of the high cost of living. Not the greatest news to newly emancipated, middle-aged parents ready for a rest.

But all too frequently, that's only the beginning. With the rise of divorce among the young, financially strapped adult children are coming home with their children in tow. It makes, for all concerned, a very full and complex nest.

For middle-aged parents, who are anxious about the welfare of their adult child and their grandchildren, refilling the nest can seem like the obvious and best solution. But the arrangement can cause major emotional and economic problems, say family therapists, who are finding more and more families caught in this three-generational bind.

The initial expectations on both sides of the three-generation family under one roof are "full of fantasy," says Dr. Carol P. Hausman, a clinical instructor in psychiatry at Georgetown University Medical School who specializes in working with middle-aged and older people in her private practice. There are high expectations that by reuniting, all current problems will ease. "The adult child wants sympathy for her plight . . . and her parents want cooperation," Hausman says.

But a middle-aged man or woman may "greatly resent this interruption in their active retirement years," she says. Their adult child's desperate call for help comes just when they are discovering privacy, marital companionship, time to travel and recreation with their own friends.

The situation can be hardest of all on single senior parents, those widowed or divorced. One widow who says she is "not in not the best physical health" cares for her two small grandchildren with no outside help. "It's exhausting," she says, "but hard to admit to my daughter or even myself, because I want to be a help."

Not the least of the problems that can arise occurs when the parents and adult child differ over the logistics of child care, says Hausman. A woman in her fifties who raised her children more strictly may find such transgressions as snacking before meals or lack of table manners a major irritant. Bedtime, says Hausman, is a common problem area: "The young mother wants her child to stay up late because she hasn't seen him all day -- and the grandparents, exhausted after a long day of child-caring, want him to retire early."

Physically, child care can prove to be more than the mid-life parent can handle. One middle-aged grandmother who says she was initially "more than willing to help" now finds herself "constantly yelling" at her grandchildren because of her lack of physical and emotional stamina.

To further complicate matters, unresolved issues between parent and adult child that may have been repressed in earlier years tend to resurface during the full nest stay. A Washington woman in her late fifties says her adult daughter at home complains that "all of our conversations are actually a lecture on how she should improve herself."

"If the parents respond to their adult child out of guilt," says Hausman, "there is bound to be trouble later on." Often, she explains, parents are using this time to "make up for mistakes they felt they made in raising their child by putting all their energy into caring for the grandchild."

Disagreement over sexual issues is a particulary sensitive problem presented to family therapists, says Roz Lehman, a Maryland social worker and speaker for "A Woman's Place: The Mature Woman Series," a counseling and activity center sponsored by Montgomery County's Commission for Women. Problems can arise over the adult child's dating, keeping late hours or staying out all night. Although the adult child needs to have a social life, she may be bothered by her parent's unspoken or spoken disapproval at her leaving the children at night or on weekends.

Both sides should consider, says Lehman, that the senior parents come from a "generation raised traditionally and still stunned by the sexual revolution and women's liberation."

"When you realize, for example, that 50 percent of women born between 1950 and 1954 will divorce, and half of those in their first seven years of marriage," says Lehman, "and compare that to their mothers' generation where 16 percent of women born between 1930 and 1934 got divorced, you can understand the tension."

Looming over all this is the crux of the problem -- money. Therapists point out that it was probably lack of money that brought the adult child back to the home in the first place, and that unless the senior parents are very wealthy, the problem will persist. "There are not a lot of families who can discuss money," says Hausman, "and middle-class families especially find it difficult."

But it is something that must be discussed. Lehman suggests that financial assistance, if given, should be brief so the adult child doesn't become accustomed to financial dependency.

At the very least, the full nest situation can be stressful; at worst, it can destroy family relationships. But there are ways to minimize the strains and make the three-generation family work together, according to counselors. Some recommended techniques for mid-life parents:

*Define expectations. "Try to talk over what your expectations are before the moving-in takes place," advises Rona Subotnik, a Maryland counselor and program director for "The Woman's Place." "Later, if everyone is living together, the family may want to use family council meetings -- if it worked for them in earlier years."

*The established boundaries. This involves defining who has what authority, and what the rules are. Family roles can become blurred in the full nest. "A young child caught between parent and grandparents may be getting two sets of rules for his behavior," says Subotnik. Equally confusing, she says, "is when a child sees his mother reverting to a child-like role around her parents."

"Parents and their adult child need to recognize and change their former roles," agrees Lehman. Once in those changed roles, everyone needs to work together in the mundane yet complex process of living under one roof.

*Set up a schedule for household chores. According to Lehman, there may be a power struggle, with the adult child resenting the parent's say in such matters as who does what. "It's hard to ask a grown man or woman to take out the garbage -- but duties need to be agreed on."

*Don't neglect your own needs. Even when the budget is tight, the mid-life couple should hire a babysitter regularly so they can spend time alone together.

*Explore other alternatives. Subotnik suggests that mid-life parents, if they can afford it, find other solutions to the full nest, such as assisting the adult child with rent elsewhere, or making financial arrangements for child care.

No matter what the solution, says Subotnik, "the key is talking things out and not plunging into something that may bring major problems.

"Family therapists are routinely dealing with three generations," she says. "People who have never had counseling before feel threatened by therapy. But once they go, they are relieved. We work with the entire family in order to keep the family comfortable and working."

"Work" is the key word. "Most parents know parenting never ends," says Roz Lehman. "It just gets more complicated and confusing."