For DELORES COURTE-MANCHE happiness and self renewal is a vacation on Nantucket Island. "But, with houses selling for $100,000 and up -- I couldn't buy a garage on Nantucket."

The Worcester, Mass., woman still can't afford a house on the island, but she has found a way to have an annual Nantucket getaway without sleeping in a stable or going broke, a way that increasing numbers of people are resorting to in the face of soaring vacation-spot and second-home costs: Time-sharing.

The concepts of time-sharing -- or interval ownership as it is also called -- is basically simple. Instead of buying an entire house or apartment outright, you buy what amounts to a share in it -- the use of the property for a fixed period annually. Courte-manche took out a $9,000 loan, repayable over five years, and purchased a total of four weeks' exclusive annual rights to a three-bedroom, two-bathroom, completely furnished Cape-style cottage in Tristram's Landing, a new time-sharing resort in the village of Madaket on Nantucket.

"It's perfect for me," she said, "I don't want a 20-year mortgage or to have to go the same place every weekend because I couldn't afford to go anywhere else." and, she added: "I'm not handy, and if I owned my own place would have to hire somebody to fix things -- this way, I get maintenance included."

Like condominiums, with which the concept is often linked, time-sharing was introduced into the United States from Europe where it has been popular for years. In this era of tight money, time-sharing has come to the rescue of a lot of financially strapped would-be vacation homeowners, but it first became popular in this country during the recession of 1973 -- as a device to bail out hard-pressed real estate developers.

With the economy severly depressed, Realtors were having trouble moving high-price properties and in desperation started selling shares in houses and condos instead of whole buildings or apartments. It not only worked, but developers quickly discovered that by selling units in bits and pieces, they could get a higher total price than by selling them outright: A one-bedroom Hawaiian condominium that sold for $150,000 could bring in as much as $250,000 when divided into two-week shares, for instance.

A hot new vacation trend had begun -- one that shows no signs of abating. There are now more than 400 time-sharing developments around the country, the number is increasing rapidly, and sales -- which have doubled annually in the last three years -- are expected to top $1 billion for 1980.

As might be expected, most time-shared units are found in traditional resort areas such as Florida, Cape Cod and in popular winter sports regions such as the Rockies and northern New England.

While time-sharing seems to be an idea well-mated to the times, there is still a good deal of popular confusion about what it is and how it operates. Does it really make sense to buy a week in a seaside condo complex rather than simply renting a similar apartment -- or a cottage -- for the same period? The answer: It depends.

By buying a time-share, you are assured of the kind of occommodation you want, where and when you want it. An important consideration, as anyone who has tried to find a water-front cottage on Cape Cod in August or a condo within skiing distance of the lifts at Stowe, Vt., over New Years's can attest. Time shares for prime periods such as these usually cost more than other shares -- but are worth more too.

Time-sharers actually have a deed of ownership to their time-share and so are free to sell shares, which typically have almost doubled in value in the past five years. The four-week share the Worcester woman paid $9,000 for two years ago is now valued at approximately $15,000, for example.

Prices vary widely around the country, but generally range from about $1,000 a week (off-season) for a small but decent condo in a good but less than posh resort, up to $15,000 a week or more for a deluxe unit in a fashionable area at a peak time -- say, either Vail or Sanibel Island at Christmas.

Like condo owners, time-sharers have to pay a management charge to cover heat, light, maintenance and property. Taxes are divided equally among the time-sharing owners of each unit and are deductible for income tax purposes. Management fees vary but are generally substantially less than it would cost to rent the time-shared unit for a similar period.

Besides monthly principal and interest charges on her loan, Courte-manche has to pay a $260 management fee for her four-week time-share, plus an extra charge of $50 to "open the house" each time she uses it. She "owns" two weeks in the fall, one in April and another in June, which means a total of $150 in opening-up costs. This adds up to $410 for a month's use, approximately what it would cost to rent a similar property for a week in high season.

"Once the loan is paid off, I'll be home free -- it's like buying a car," she said. Time-share purchase payments are fixed, but management fees increase to reflect inflation and increased operating costs. They keep going up, in other words. Courte-manche's management maintenance fee has risen nearly 30 percent in two years.

What happens if for some reason you can't get to your property during the period you own, or simply need a change of scene and want to go somewhere else during that time? It's your share, and you can leave the property empty or loan it to a friend if you choose, but the management can usually arrange to rent it, charging a fee for the transaction, and very often you can "swap" your time-share with someone else -- maybe someone on the other side of the globe.

Virtually all time-share developments are affiliated with a vacation exchange service which arranges swaps for owners. The two largest exchange services, Resort Condominiums INTERNATIONAL (RCI) and Interval International, represent properties in more than two dozen countries.

Indianapolis-based RCI, oldest and largest of the exchange services, was incorporated in 1974 and has some 100,000 members. Users pay an annual membership fee of $36, for which they receive an RCI directory and other publications along with membership services such as rental car discounts. There is an exchange fee of $28 per week and exchangers have to give 60 days notice and offer the RCI space bank of choice of two resorts in two different regions as well as several different possible swapping dates.

Interval, with 85,000 members, has headquarters in Miami and considers itself a travel and leisure club with vacation exchange "the key service."

For a $35 annual fee, members also receive benefits such as travel insurance, a luggage tracing service and a variety of discounts. There is a flat $35 exchange fee, regardless of the number of weeks involved.

Exchange services claim successful exchanges about 80 percent of the time -- but usually insist on swapping similar-size units for the same reason, a one-bedroom condo in Florida in May for an apartment of the same size in the south of France in spring, for instance. This means that an off-season time share may be inexpensive to buy, but relatively difficult to swap: You may love Vermont in November, but do you want to go to Colorado then?

Although time-shares have generally been appreciating rapidly in value, developers and Realtors are cautious about recommending time-sharing as an investment -- although they usually do stress that it is a hedge against inflation. "I see it as a vacation alternative. You are buying a vacation, not real estate, and should consider time and location, not just trading value," said Carl Burlingame, publisher of Resort Time Sharing Today, a magazine of the time-sharing industry.

Burlingame feels that time-share buyers "don't always take the realistic approach" when making purchases, particularly about maintenance fees. "I wouldn't buy into a property where the maintenance charge was too low," he said. "There are apt to be a few surprises when the owners' association takes over." (As with condominium complexes, time-sharing developments are usually turned over to a members association when approximately three-quarters of the units are sold.)

Since time-shared units have not one but usually many owners -- all with different habits and attitudes toward property -- the wear and tear is considerable and upkeep often costly. Anything less than $100 a week for maintenance of an average one-bedroom condominium in a resort area is probably unrealistic, according to Burlingame.

"Quality of management is the important thing to look for, particularly in a new development," said a veteran New England Realtor, who advises would-be time-share purchasers to check developers with the attorney general's office and the Better Business Bureau before signing an agreement and putting money down. There have been cases of developers running out of money and leaving luckless owners with over-priced shares in an unfinished -- or underfinished -- development. (The quality of construction is also a factor to be considered.)

Besides interval ownership, there are some variations on the time-sharing theme. Under leasing arrangements, you don't actually own a share in a property for a specific period but only lease it for a fixed period of years. Leased time shares are usually about 20 percent cheaper than interval ownership shares, but, of course, can't be sold. There are also "right to use" of "floating lease" agreements, under which you don't purchase use of a particular unit for a stated calender period, but only buy access to one of a certain standard or category during a specified season -- say, a one-bedroom, waterfront condo in Miami in January.

The type of time-shared units available is becoming more varied, too, although surveys indicate that most buyers prefer two-bedroom condominiums in a development built specifically for time-sharing.

An introductory brochure on time sharing is available by sending 50 cents and a stamped, self-addressed envelope to Resort Time Sharing Council, 1000 16th St. NW, Washington, D.C., 20036. A directory of existing U.S. time-share projects, "Buyers Guide to Resort of Time Sharing," priced at $7, can be obtained by writing to: CHB Company, Box 184, Los Altos, Calif. 94022.