The sand is warm between your toes, the waves lap gently against the shore, sailboats dot the horizon, the scent of suntan lotion wafts seductively on the light breeze. Only three things could possibly mar this idyll:

1. Horse flies. 2. It's not much fun if you're alone. 3. It has to end.

There's not much you can do about 1 and 3. As for 2, a communal beach house is an answer, and it also provides a way to stretch the illusion of a vacation from Memorial Day to Labor Day, sometimes beyond.

Washington, it's no secret, is the kind of place where people give you their work number before their home phone, where the people you meet are primarily through the office and even if you do run across someone you might like to know better, you don't have time for them because you're working. There's also the free air conditioning at work. Without a spouse or child to demand that you occasionally stop working and smile at them, you may have found that the only way to not work is to leave town.

Thus the beach-house phenomenon: Groups of friends, acquaintances or even total strangers -- usually single, but occasionally including established couples -- share a lease, a roof, companionship and the outside shower.

The house at the shore is rented for the entire summer and sublet for as many weeks as possible from Sunday night to Friday night, with the weekends reserved for the group's own escapes. The house offers easy, informal sociability, and there are the long stretches of beach for those who seek solitude.

The Friday-afternoon ritual becomes automatic: Collect bathing suit, beach towel, suntan lotion, cooler, Frisbee, friend (if available); fill car with gas; leave city, work and mind (as the saying goes) behind as the sun sets over the Chesapeake Bay Bridge.

That exodus went on all summer long, and tomorrow many Washingtonians who recognize "coast" as the operative word in the term "East Coast" will set out on the official summer's-end expedition to points along the Delaware and Maryland shore for Labor Day weekend.

And before their tans have faded -- or even before they leave the ocean on Monday -- they'll be making plans for next summer's getaways, looking for new members for established houses or forming new ones as the tide goes out on this year's.

The same people who trudge through Dupont Circle in drab khaki raincoats over dress-for-success suits and sensible shoes undergo a remarkable transformation when they exchange their clothes for bathing suits. As one avid beach fan says, "I'm a completely different person at the beach. Everybody's different at the beach."

The group beach house has become such a social phenomenon in Washington that a newcomer to the city reports that one of the first questions she was asked upon arriving here was whether she belonged to a group or would like to join one.

A 30-year-old attorney who has lived here for five years first began going to the beach as a guest in one house. He was quickly lured into joining a house the next year, not, he says, "particularly to meet people or work on my tan, but because it was an adventuresome thing to do on weekends. And when you can get out of the city, the weekends spent at home are that much more enjoyable."

Some groups have histories that go back as far as 20 years. Sometimes one member of the group owns the house; sometimes the group finds a house to rent, either from an individual or through a Realtor. More than one veteran stress the importance of actually seeing the house before signing up.

One longtime beach-house organizer recommends that the group be started with a nucleus of four or five friends. "Then you start talking," to fill in the rest of the shares.

Before you start planning next year's escape, consider the range of choices. Rehoboth has a fairly staid population of families who have made a tradition of summering there (but also offers such preppie watering holes as the Rusty Rudder, the Summerhouse and the Bottle and Cork, where sunglasses are recommended to dim the glare from green pants, pink shirts and blond hair).

Dewey Beach, just to the south, mainly houses the 25-to-35-year-old professional Washington crowd ("fairly raucous," says one weekend resident). Lewes, just to the north, offers rambling houses, roughly hewn dunes and nonexistent night life. Further south are Fenwick Island and Ocean City, all within a three-hour drive.

Compatibility, veterans agree, is important in constructing the group. The wild-alcoholic-orgy tales that make their way back over the bridge are not everyone's idea of a good time. One beach-house owner recommends a balance of men and women to keep the "zoo" element at a minimum.

There are plenty of group houses, however, for which a typical weekend consists of a Friday-night stroll down Rehoboth's civilized boardwalk, perhaps a pizza from the Grotto, a game of Ms. Pac-Man and a good night's sleep. On Saturday, after a leisurely breakfast (Entemann's coffeecakes disappear by 10 a.m. from the corner market), members disperse for the beach. If you have energy, there's Frisbee, volleyball, whiffleball. If you don't, that's okay.

Whatever group members and their guests who have come to the beach that weekend may or may not convene for a communal dinner of fresh seafood, scattering again afterward for movies, bed, an Agatha Christie mystery on the screened-in porch or a couple rounds of miniature golf.

The few responsibilities -- shopping and cleaning -- are easily shared. Frustrated cooks tend to be in their element when called upon to feed a houseful of relaxed, sunburned, hungry people.

"It's a college atmosphere for a weekend," says one unattached participant. "You don't have to go through making elaborate plans to make sure you have something to do on the weekend. You have an instant group of people to do things with."

The tenor of that social life, however, can change with little warning. The core people in one group this summer set out to recruit only singles. At the customary pre-season party the members all adamantly professed a love of independence, commitment to their roles as free spirits, enthusiasm over the prospect of tennis round robins, volleyball games, charades, bonfires on the beach . . .

On opening weekend six of the 10 "free spirits" showed up with "friends" who quickly became permanent romantic appendages.

Love is easier at the beach.