KITCH' EN. 1. a room for the preparation and cooking of food. 2. `an entertainment and social center for the entire home.'

THERE was a day when the kitchen was mother's own turf. Yes, you could visit, but no, you couldn't live there. Well, not any more, and probably never again.

Ask Mary Fisher Knott. She's been poking around many a kitchen for some 22 years. Not just scouring pots and kneading dough, but down on her hands and knees measuring, tearing down walls, and drawing up blueprints.

Mrs. Knott has designed over 3,500 kitchens in those years, and when she questions whether you really need that combination electric can-opener and floor polisher -- think about it.

She and the publishers of Home magazine have compiled a colorful, helpful book, ``Creative Ideas For Your Home Kitchens'' (Knapp Press, Crown Publishers Inc., $24.95), with photographs showing the do's of kitchen design and text telling about the don'ts.

``Years ago, no one wanted to be seen in the kitchen; now it's an entertainment and social center for the entire home,'' she said in a recent interview.

``People don't just cook in kitchens any more. Often there's a TV, or even a place to sew or do flower arranging, and a place for kids to study.''

The big new item, though, is the computer. ``In five years' time, don't be surprised if you're looking for a space to put one [in the kitchen]. It's something to think about,'' says Mrs. Knott.

One point Mrs. Knott emphasizes: If you plan to change your kitchen, pay attention to your own specific needs. ``One thing we always do is to go over the various foods our clients like to cook. If they do a lot of baking, they need that extra counter with a marble top, or a separate space for making pasta. Preferably away from other working areas.''

She sternly warns people not to be intimidated by designers or contractors.

``After all, you're the one who knows what you use your kitchen for more than anyone. ``Sometimes a contractor will say, `You don't want this' or `you don't need that.' Well, that may be their way of saying that they can't get it for you or they never heard of it, or they don't want to do it.''

First things to consider in planning are layout, traffic patterns, and counter space. And you've got to be realistic.

``The most frustrating thing is when someone takes you into a kitchen the size of a broom closet and smiles, saying, `We want to be able to eat here, too.' ''

Except for those unreasonable requests, it's really easier to design a small kitchen than a large one, insists Mrs. Knott.

One thing small kitchens have is accessibility to everything.

``And you can always add pullout boards that can be used as temporary tables or counter space. Just be sure you don't set them up in a traffic area or over the drawer where you store your silverware,'' she warns.

Even a simple thing like lighting is important. ``Sometimes I'll go into a perfectly lovely kitchen and I'll hear, `Oh, we just hate this room.' It's usually on the northwest side and it's just dreary. All it needs is good, bright, imaginative lighting.''

Above all, a kitchen must ``work,'' Mrs. Knott says. ``You'll probably spend more money on the kitchen than any room in the house -- thousands of dollars! And it's unforgivable to blow that kind of money if it isn't functional. And if it isn't functional, it isn't worth a nickel you put into it.''

The thing most lacking in a poorly designed kitchen, says Mrs. Knott, is common sense.

Some points to consider are these:

Don't try to cram too much equipment into too small a space.

Purchase appliances that fit your specific needs as a cook. Single people may need more freezer space and less refrigerator space.

Think of the future. You may want a computer. Not just for recipes but for working on the family budget, records, or even setting up a security system.

Keep up with new products.

Consider the unexpected. Two dishwashers sound excessive, but they are no longer uncommon in large households where people entertain a lot.

Study kitchen traffic flow, and make sure new appliances and counters are not in the way.

Keep your options open. Listen to others but decide for yourself.

Think of a kitchen as a work center for the family, not just a place to cook.

If you don't have a lot of money, find out what you don't like, what your specific needs are, and go from there.

Mrs. Knott has worked with a lot of difficult people and situations. ``The most difficult is what I'm working on now -- my own kitchen. I'm my own worst client. I've got four plans in front of me and can't decide what to do!''