The tiny Atlantic resort of Bethany Beach, Del., has long been appreciated by the the peace-and-quite set for its short boardwalk, relatively inexpensive cottages and small apartments. Recent years have also seen the addition of new town houses at the ocean front and larger single-family homes.

But since the early 1970s, the on-going development of a major high-rise complex called Sea Colony, orchestrated by Washington builder-entrepreneur Carl M. Freeman, has begun to change the face of the once somnolent area.

If consistency of sales and amenities are the yardsticks, Sea Colony has been doing well - even weathering the 1974 downturn when a gas shortage combined with the mortgage money crunch to depress all oceanside realty markets. Now more than 660 Sea Colony units are sold and one new building is virtually sold out. Sales for a building now under construction have also started out strong this summer. Sales manager Patrick Rhodes, a young Washingtonian who has witnessed the Sea Colony emergence, said that the new units average about 1,100 square feet with two bedrooms and a den in a range from $48,900 to $74,900, depending on location.

Partly to accommodate the new development, a new sewage treatment plant is being built at the resort. Hundreds of oceanfront Sea Colony owners and renters and all the folks along the beaches there have been watching the work ships just off the shore, where treated wastewater from the new plant will be funneled into the sea after the big pipeline is completed.

Sea Colony, which also has an enclave of contemporary town houses and a big bevvy of tennis courts, has room for one or two new buildings to the south, as well as land to the north of Bethany Beach, where another high-rise complex may spring up in the 1980s.

Also to the north of Bethany, between the large houses in Sussex Shores and those in the Cotton Patch section, a group that includes Ocean City, Md., realtor Kenneth Baker has recently subdivided a 12-acre tract called Sea Dell Estates into 43 lots, some of them fronting on the ocean. Fourteen were sold in an eight-day period for a total of $746,000. Baker said only single-family houses will be built, along with a tennis court and playground. Lots, which are about 16,000 square feet, are served by the local water system but are not hooked up to the sewage system. Septic tanks are not uncommon on most developments outside Ocean City, Bethany Beach and its neighbor to the north, Rehoboth, Del.

In Rehoboth, where the fabled Henlopen Hotel has been rebuilt as an apartment-hotel condominium, major new construction has tailed off in recent years. Nearby Dewey Beach still has its popular cottages and town houses around a few block-siezed estates near Silver Lake, in addition to low-rise apartments and cottages.

One main difference between Rehoboth and Ocean City is that the Delware resort has a larger permanent population. Streets two, three and four blocks inland from the ocean are much like those in Vienna, Va., or Kensington. And new single-family houses still are being built on the site of what once was a golf course. But the principal thrust of Rehoboth's prestige has to be the developments at Henlopen Acres and, more recently, North Shores.

The "Acres" was started as a vacation retreat in the 1920s by Wilbur Sherman Corcoran, who weathered the Depression and then reacticated in the late 1740s - when quarter-acre lots went for $3,000. Most of the construction of those prestigious "Acres" houses occurred in the 1950s and 1960s. Unbuilt lots are privately owned now; one was recently sold for about $50,000, according to a Henlopener. House resales generally range in the exclusive $150,000 to $250,000 category.

Not unlike some of the lesser-priced little communities near the Del-Mar beaches, the residents of Henlopen Acres are apparently becoming more retirement-oriented.

Down at Bethany, competing realtors Thomas Crowley and Joseph Tansey pointed out, along with salesman David J. Long of East Coast Resorts, Inc., that many sales in Bethany West and other communities of single-family houses have been to the retirement-minded buyers. And they're not all from Washington, Baltimore or Wilmington. "More and more Pennsylvanians are showing up," Long said.

Crowley, like Tansey a former Washingtonian, has been living with his family in a older, restored house near the canal in Lewes, Del., at the mouth of the Delaware Bay. But now he plans to move south to the Bethany area, where Crowley, Evans & McCaffrey sells, builds and develops.

But Lewes, a settled community with a ferry stop and a hospital, will continue to be home to Dennis Forney, editor of the Whale, a weekly newspaper there.

"I restored an older house and there are a lot of new owners doing it here," said Forney. Pilottown Road, which runs along the mouth of Delware Bay, has some fine old houses, as does the canal area in Lewes.

As the summer wanes, year-around residents of the Del-Mar ocean area look forward to the more solitary enjoyment of their turf and surf. Many restaurants and shops will close in October but the real estate office will remain open through the winter.

C. F. McNeil, Anderson-Stokes sales manager in Rehoboth, said that the summer season demands emphasis on rentals up and down and around the beach and that some contacts are thereby made for selling. "But the fall and spring months are best for selling properties," he added.

However, realtor Tansey said that he's been averaging a couple of resales a week during the summer at Sea Colony. Other realty specialists also reported a better-than-average season for selling.

Meanwhile, housing economist Michael Sumichrast returned from the beach to report that he had discovered some available inland building lots in the $5,000 range. And other experienced observed noted that some nice beach area houses, although not on beachfront, houses are available in the $40 and $50s. Sales have improved because financing now is more readily available. But there has to be some give in asking prices because some of the dwellinngs have been on the market for a few years. CAPTION: Picute 1, The Sea Colony development of Carl M. Freeman at Bethany Beach, Del, is a major force in that resort's real estate market.; Picture 2, The area of Rehobeth Beach, Del. is also having a good sales season. Photo above by Ken Fell - The Washington Post; Picture 3, Among the new homes in the Bethany Beach area are these at White's Creek Manor, priced in the mid $40,000s. Development is on waterway near bay., Photo by Jack Hayes