WHY ARE SO many our gently reared daughters determined to discard their comfortable suburban backgrounds to share an expensive old mansion in a deteriorating neighborhood with strangers?

Until recently I looked on this as an interesting social phenomenon, intriguing but unfathomable. But this year I helped my own daughter move into two such houses in sucession. I also ended up asking her to reply on paper to a series of questions I posed, and I now can reassure my contemporaries: I have seen the new culture, and it works. While different, it presents no cause for alarm.

I have, in fact, come to know a particular communal house of this kind rather well. It was once a handsome house-still is if you factor out the trash in the tiny fenced front yard, the need for paint, the damaged iron grill on the window and the deterioration of the neighborhood. A grande dame down on her luck, the house sits almost within sight of the Capitol and is home to my 23-year-old daughter Joan and three other young people.

Until a month ago the young people were unknown to my daughter and, for all I know, to each other. She answered a classified ad to share a rent that adds up to almost as much as landlords get in the most expensive Washington suburbs.

The pragmatic way of beating high rent is a menage of convenience. Shares are thus advertised publicly, though acceptance for aplicants is far from assured. A prospective shareholder must be inspected and a blackball is not unusual, no reason given. ("We like you, but not quite as much as the one we took.") Does the aplicant have the right sort of public interest job? Smoke? Eat meat?

My daughter works in a discount bookstore and is looking for a job in journalism (both acceptable). She lives on the third floor of the old house in a large, handsome bedroom with carved moldings, a lovely old fireplace (non-working) and a floor-to-18-foot-ceiling window looking out on the neonlit bar and grill across the way.

I have outfitted her with a typewriter table, a small painted chest of drawers, a lamp that she keeps on the floor for want of a table, and a deck chair over which she has thrown a Mexican rebozo. All furniture in these houses is automatically draped with whatever is handy, for reasons to escape me.

There is nothing else in this room but a mattress, which she has put on the floor under the window. The mattress appeared one day from the basement to take the place of the sleeping bag she used for the first week. The room, with her toilet articles on the mantlepiece, has the look of a house in which the movers are awaiting the arrival of the family with the furniture.

I have nodiing acquaintance with her current housemates, and I like them. Like her, all work nearby, share the utilities and srupulously take each other's phone messages. Occasionally they cook together, but more often they go their separae ways, dining in places with names like "Food for the Soul" or "You!" They have met recently to consider the problems of rats in the front yard. Nothing about them reminds me of the simple-minded fraternity antics portrayed in TV's "Three's Company," which is supposed to reflect today's communal living.

It was with the diffidence required of all mothers inquiring into their daughter's mode of living that finally got up the nerve one day to put some questions to Joan about life in this house on 17th St. She agreed to tell all. Following, then, are a mother's questions to her communal-living daughter and the daughter's replies.

If this is a safe neighborhood, why are the iron bars on the windows so twisted?

No one ever claimed the neighborhood is crime free. To me it's just better to learn to survive in a tough neightborhood than to retreat to one that's totally safe and isolated. I love being able to look out the window and see the local hangout/pizza place with its neon heart flashing on and off. I like the feeling I am in the middle of things where whites, blacks and Latinos mix. Every time my mother and I meet she hands me a fistful of clippings on who got raped nearby. In her heighborhood, people spend all their time on their own private gardens, trying for greener grass than the lawn next door. I don't want that.

Do you realize it would be cheaper in the suburbs?

We don't go into group houses to save money. That would be a pretty sad reason. Anyway, heating those Victorian houses in the winter gets expensive. But sharing a house means you can come down to the living room Sunday morning in your bathrobe, have someone to read something funny in the paper to, have someone to go with you afterward to an art gallery.

I came from the suburbs. This is a warmer, wider, more real, better.

How do you find a house with a vacancy and how can you tell on a brief encounter that you want to live there?

As with jobs, word of mouth is the best way because the competition is so stiff. After that, local bulletin boards in the vegetarian restaurants and plant stores are the best bet - thoough they often stay up months after the new resident is happily installed. Finally, in a place like Washington where group living is well established, the daily papers have columns for houses to share. I think it helped that I worked in a bookstore and could get employee book discounts.

It's hard to tell from those sterile, two-line ads who you're getting in with, but after months of searching, I noticed a pattern. There are basically two kinds of group houses-hotels and families.

One place I was interviwed was a beautiful old building on 16th St. that at one time housed the Saudi Arabian embassy. It was an enormous place with large Greek pillars in the living room. Seven people lived there. If you were on the top floor. it would be easy to forget about your housemates below. But to many people, especially if they are living in a city for the first time, people-support is important. Some houses I know eat together every night, have weekly meetings and are very careful who they let in when a vacancy comes up.

Family houses are typified in an ad I saw in a vegetarian restaurant: "We give each other a lot of support, so don't answer this ad if you hate being hugged."

Do you really not notice all that dirt?

What dirt? If it's not visible, it doesn't exist. I have two jobs, do volunteer work, go to community meetings and concerts, and do not wish to spend every spare minute making the house look like an ad for floor polish. Which doesn't me we don't care about how it looks. My room is lovely and I appreciate that. If I were looking for convenience and a place that was easy to keep clean, I would rent an efficiency.

Are these arrangements always a-sexual?

Definitely. It is an unwritten rule that you don't get involved with someone in your house. It gets too messy. If you move in as lovers, that's different.

How do you know one of your housemates isn't Jack the Ripper, who has been controlling his impulse of late?

I suppose if you're going to be that untrusting about it, this is a possibility. But I've never heard of it. If a strange male voice comes on the phone in answer to your ad and says he has a lovely little apartment, rent negotiable, obviously you sau no thanks.

Of course you do run into some odd circumstances. One place where I went announced there there was a lesbian couple living there and waited for me to react. At another house, all students, I was advised that I might feel out of place if far-left politics didn't play a big part in my life.

Where did that mattress come from?

I don't know where it came from, but I'm delighted. It means I don't have to spend time and money shopping around for one. Probably one of the people who use to live here left it because they couldn't be bothered to cart it out. Group houses are, by their nature, transient. I don't want to accumulate furniture because I don't know how long I'll be in this city. The mattress was a happy surprise.

I don't own a stick of furniture except my stereo, and that's broken. When I get a little ahead, I'll have it fixed. Meanwhile, the house has one and I provide records. My mother's generation was hung up on things . The people I know are not.

How do you share the kitchen chores?

Pretty arbitrarily. Guilt, hunger, or (occasionally) a desire for cleanliness eventually propel everyone to do something. Admittedly, dishes have been known to pile up in the sink. We all have Better Things To Do than wash and put away dishes. But by Sunday afternoon the counter and sink are always visible to the naked eye. I'm not sure how this happens, as I tend to be away on Sundays.

In our house we buy food together and sometimes cook together. There is no such thing as private food, so if you have something special you don't want to share with three other hungry people, you'd better eat it fast or squirrel it away guiltily in your room. Grocery shopping is not assigned, but it usually gets done, especially when we run out of something vital like peanut butter or toilet paper.

If most people are sound at dinnertime-which doesn't happen often/and someone is cooking, he or she will make enough for everyone. I am a terrible cook, so I have managed so far to eat other people's meals and stick to washing dishes.

Is a diet of trail mix, yogurt and mung beans really adequate to sustain life?

My mother is convinced I never eat a sqaure meal except when I come home. (She can't forget I once had a picnic lunch of choclate bars, Brie and wine). A meal, she believes, must have meat and potato and vegetable. In group houses, even if the people are only "part-time vegetarians," you quickly learn about complementaty protein and how to make split pea soup. We eat everything, but peanut butter and jelly is a good fallback, especially on protein-packed granola bread.

At the last house I lived in, there were seven of us and nobody had the same schedule. So we ate separately and bought our food separately. Ther were two refrigerators and each of us had a shelf and you got the absurd situation fo four half-pints of milk in one refrigerator. I think my housemates thought I was a little strange because I didn't eat out of cans, but regularly cooked up a mess of bulgur and fresh vegetables. It seemed to be news to them that you could buy fresh broccoli at the Safeway.

A lot of the group houses in my neighborhood are vegetarian. At some of the places where I was interviewed, the residents inquired anxiously if I planned to bring any red meat into the house. One friend of mine had a very hard time finding a place because she not only eats meat but smokes. She finally got an efficeincy in an apartment full of 9-to-5 hamburger eaters.

Are you, well, happy?

Yes. Are you? CAPTION: Picture 1, no caption; Picture 2, Joan Mooney in her third-floor bedroom. By John McDonnell-The Washington Post