EVERYBODY KNOWS how to behave in another person's house when the owner is looking, Miss Manners trusts. Now it is time to learn how to behave in someone else's house when the owners is not at home.

This is not an etiquette lesson for housebreakers, although goodness knows they behave badly enough. Miss Manners is addressing this to house-sitters.

House-sitting ought to be the ideal arrangement between the haves the have nots. People with property usually lack the confi dence to leave it unguarded. People without property usually lack property. The two groups seem made for each other.

What prevents house-sitting from being one of our major solutions to social problems is the basic distrust that the landed have for the lacklands. People who are temporarily vacating their houses tend not to match themselves up with those who are simultaneousl bein g evicted. They choose, instead, people who were born members of the properties class, but who are temporarily propertyless, through restlessness or youth.

But this identification of class presents a difficulty. House-sitters who are friends of the house owners, or offspring of such friends, confuse house-sitting with house guesting. These are two different occupations, although the same person may perform both in the same house at different times.

A house guest is supposed to make his presence more desirabel than his absence. A house-sitter need only be more desirable than those whom his presence is intended to discourage.

However, he must keep the odds on his side. If you leave your house empty during your absence, it is possible that it will be undisturbed, and you will find it just as you left it. If you could be sure of this, no one, except those with heavy plants, would need a house-sitter.So the amount of breakage, disarray and snooping done by the house sitter should not exceed the amount likely to be done by housebreakers - after it is averaged in with the possiblitity of nobody's touching the house.

Neatness only counts when the owner is back to see the state of the house, and therefore house-sitters do their straightening on the day before the announced return. They should bear in mind that travel plans tend to get changed and home owners are given to walking in without notice as if they owned the place.

The rules of breakage are not the same as for a house guest, who need only replace small items if the host fails to protest after the second offer. House-sitters must always replace what they have broken, in kind or in money. What is more, they must confess.

Invasions of privacy must only be confessed if they are going to come out anyway. If you have ruined the house owner's reputation with his neighbors or the police, you might just as well tell your side of the story in the most favorable way. If he is likely to meet people you have entertained in the house, their names should be mentioned. The fact that you showed them all the private rooms need not be, as this can be classified as getting lost on the way to the bathroom.

But under no circumstances should a house-sitter confront the house owner with information gleaned from the house owner's diary. MISS MANNERS RESPONDS

Q: My husband and I have been married almost two years. For personal, as well as professional reasons, I didn't change my name when we married. We now have a child whose last name is a combination of my last name and my husband's, hyphenated.

My problem: Neither of our families fully approves of these circumstances, and therefore not only call me by the wrong name, but also our son. How can I tactfully tell them I don't want our child growing up not knowing his or his mother's correct name?

A: The important thing is for your child to grow up knowing that, as we live in confusing and changing times and probably always will, it behooves us all to be flexible and tolerant. Your families are admittedly be inflexible, but this gives you the opportunity of setting an example of tolerance. Your son will, no doubt, find out his name some time before he has to fill out his college entrance applications.

Q. I will be moving into a new house soon and would like to have a housewarming party. I was told that it is wrong to give yourself a housewarming party. Would it be better to send out open house or housewarming invitations? Please answer soon because we are about ready to move in.

A: Miss Manners is answering as fast as she can, and hopes that your movers, painters, electricians and plumbers show you the same consideration.

You were told wrong.A housewarming is correctly given by the people who own the house. You may call it whatever you want. Miss Manners heard of one given by the new owners of a house in which a famous sex scandal had taken place, and it was called a housecooling, because the house was hot enough already. All housewarmings are also "open house" - which means that they are given in a cocktail party sort of way, but generally for families rather than just adults - although not all open house parties are housewarmings.

Q. My son-in-law is Mr. Joseph X. Doakes IV. His predecessors, I, II and III, have died. His son is Joseph X. Doakes V. In social matters, what is the acceptable reference to these two family scions: Jospeh X. Doakes Sr., and Joseph X. Doakes Jr., or the tradition English custom, IV and V?

A: As your relatives seem to be American, Miss Manners questions why they want to go around souding like kings or popes. Actually, Joseph X. Doakes V sounds like two popes - Joseph X and Doakes V. American usage limits itself to enumerating the bearers of the name who are alive, with the senior member using the name along. His son uses "Junior," his son, "III," and so on. When the ranking Joseph X. Doakes dies, his son passes the "junior" on down to III. "Sr." is not used, except by a widow wishing to distinguish herself from the daughter-in-law who is married to the ranking name bearer, simply "Mrs. Joseph X. Doakes."

Q: It infuriates me to see people using the express line at the grocery story for huge baskets piled with food. The sign plainly says, "10 items or less." Is it all right if I explain that to people I see with more in their carts?

A: Only if you explain that sign to Miss Manners first. Do a dozen oranges count as one item or 12? Is there a difference if they are in one mesh bag or running around loos? If there are three cans of soup for 69 cents, and three clam chowders count as one item, what about two clam chowders and one chicken with stars? Also, why doesn't that sign way "fewer" instead of "less"?