It seems to be increasingly these days. Children grow up, leave home, work, and then move back home. Sometimes it's after five years. Sometimes it's after 25 years. I've been there as a parent … a few times. Most adult children who move home do so as an expedient.
Although sometimes the homecoming son or daughter is accommodated by children and / or a spouse, which makes the arrangement more complicated, I'll assume for the present the returning adult child is single, and the parents are together.
The typical agreement among the three is that they will all get along and treat one another with respect. Each will contribute financially according to means. Obviously, the financial contribution of the returning offspring may be quite limited.
Since they are three adults living together, they agree to chip in and do their share of the usual chores around the property. They also agree that if things do not work out for any reason the son or daughter will make their own arrangements to live elsewhere.
"Not working out" is intended to mean that one or more of the parties becomes unhappy with the arrangement and can not get past that. Unhappiness could arrise because the adult child's "occasional drink" turns out to be serious drug or alcohol abuse. Criminal behavior, (theft, producing drugs, holding stolen property, etc.), someone feeling abused, or the parents fighting with each other could be other sources of unhappiness.
Unfortunately, this very sensible agreement is seldom written down. Then, if things deteriorate, instead of looking for sensible solutions, each refer back to his or her recollection of the agreement: "but you promised …" Everyone seems to have forgotten the escape plan.
The dynamic of the typical deterioration is almost universal. In rare cases it may be avoided if all parties are very conscious of it in advance and are very vigilant.
So what is the dynamic? It usually starts off well with everyone having the best intentions to live as an adult couple with another adult. However, they have never done this before with one another. But they have years of experience of living together as parents and a child.
The adult son does something that he would have done as a teenager at home, but has never done as an adult on his own. Or one of the parents say something that they said frequently when their daughter was a teenager at home, but would never say to a stranger who was staying with them as boarder.
A teenager's psychological work is to be somewhat defiant in preparation for leaving home. The parents' job is to launch their teenager into adulthood.
The parents say "Let us know when you'll be home." They may be asking for the common courty that they give to each other. But the adult son or daughter experiences this as an attempt to control. Memories of teenage curfews are triggered. In unconscious defiance, they do not give the courty they would have extended without a question to a same-aged roommate.
It takes great restraint for a parent to not act like a parent when you're 25-year-old or 45-year-old offspring is acting like a teenager. I'm not sure I ever got the knack of it. It is even more of a challenge for the less mature adult child to stay adult when the parents are acting like parents of a teenager.
For the sake of everyone's sanity, always have an escape plan. You could even post it on the refrigerator.