"For Sale" signs and skeletal frames of new houses are as much a part of the scenery here as pounding surf and salt marshes. This long, curled peninsula is in the clutches of an unruly building boom that is carving up sleepy towns that have part-time governments, loose zoning laws and no long-range plans for roads, water, or preserving a way of life.

"We've lost a lot of what people come here for: a rural-character town, a different way of life. What we're developing into is a suburb of Boston," said Jean Thomas, a selectman in the town of Mashpee. The town's year-round population has increased 40 percent since 1980 and its available housing by52 percent.

The peninsula's permanent population of 167,000 is growing faster than any other region in New England -- more than 50 percent between 1970 and 1980 -- according to the Cape Cod Planning and Economic Development Commission. The commission says that more detached, single-family homes were built here in 1985 than anywhere in Massachusetts. More than 16,800 new housing units were approved from 1980 to 1984, another 5,000 in 1985 and 1,095 in the first three months of this year.

Investors, retirees, successful baby-boomers and Boston commuters have been lured here not just by the sea, but by falling interest rates and some of the state's lowest property tax bills.

" 'If it's for sale, let me buy some.' That's the reaction we seem to be getting," said Armando Carbonell, the commission's director.

"It's a scary situation," said F. Thomas Fudula, Mashpee's first and only planner. Fudala said he recently was offered $60,000 for an acre of land in Mashpee that he bought last August for $23,000.

"Builders in town are desperate to build," said Fudala, 35, who was hired by the board of selectmen in 1984 in lieu of declaring a construction moratorium. Fudala, a graduate of the Harvard School of Design, works 60 hours a week for $27,000 a year. He has reviewed 3,000 lots and condominiums during the last 18 months, with help only from his secretary. He said his ulcers are getting worse.

Building booms hit the Cape in the late 1960s and the early 1970s but were interrupted by economic downturns, said state Sen. Paul Doane, whose ancestors arrived on the Cape in 1634. Five years of sustained growth "at a pace that is unrivaled" finally made the peninsula's 15 towns examine "what we're doing to our land and particularly to our groundwater," Doane said.

More than 15,000 public and private wells have been sunk into a single aquifer, the Cape's sole source of water. Groundwater is particularly vulnerable to contamination because pollutants flow freely through the sandy soil.

Barnstable County health officer Stetson Hall said that well-water samples in Mashpee and nearby Falmouth show contaminants, possibly from a nearby Air Force base, the town landfill or septic tanks. Hall said that 200 residents in two Mashpee neighborhoods have been advised not to drink the water.

He said 30 Cape shellfishing sites are closed because of high bacteria counts. Even golf-course greens are sampled to determine if their fertilizing chemicals are polluting the water supply. Consultants in Hyannis are studying public wells downhill from a sewage plant. In Falmouth, the town dump and the sewage treatment plant were built upstream from a main water source. But, Hall said, the Cape's overall water quality remains good.

"I haven't given much thought to it other than buying some bottled water," said Henry Hooten, 47, a high school teacher from suburban Boston who paid $95,000 for a town house in Mashpee in January. Asked why he bought the house, Hooten said that it was an investment and pointed to a sign saying the properties were selling for $129,900.

A home on a pond in South Yarmouth costs $375,000, and a three-bedroom "retiree's-dream-come-true" costs $134,000, according to local real estate advertisements.

A house in the dunes in Chatham, a quiet seaside town at the Cape's elbow, was on sale for $700,000 and a "wonderful family home in a nice area" of East Harwich costs $129,000. In the resort community of New Seabury, a small Nantucket-style condominium overlooking the ocean costs about $200,000.

Michael Frucci, executive director of the Cape Cod Chamber of Commerce, said that one of the "terrifying" aspects of skyrocketing real estate prices is that young people now have "damn little chance to buy a new home."

Rental units on the Cape decreased by 35 percent in 1984, according to the planning commission, and the state's Commission Against Discrimination plans hearings to determine if some of the Cape's realtors are discriminating against low-income renters.

The Cape's $1 billion tourist industry, dependent on workers to staff restaurants, motels and gift shops, is considering providing housing for workers priced out of the market, Frucci said.

Meanwhile, much of the Cape's west end has become a bedroom community for Boston. About 10,000 Cape residents commute up to four hours to and from Boston every day. There is talk of commuter railroad service, and some towns are looking distinctly suburban.

At 6 a.m., the Park 'n' Ride lot on Rte. 6 is packed with commuters taking vans and buses to Boston. Highways on the Cape's south side are clogged with traffic and lined with subdivisions.

A stronger sense of "old Cape Cod" permeates the north side, although many of the captains' mansions in the historic district on Old King's Highway have been turned into inns and antique stores.

The grounds of the Nickerson Mansion in East Brewster are covered with clusters of luxury condominiums, but the great stucco house built by millionaire Sam Nickerson in 1908 will be preserved.

"Sam would have wanted it that way," a brochure says. The mansion will be used as a conference center.

Some Cape towns have adopted zoning laws that require an acre of land for each housing unit. But a state law exempts land from zoning changes for eight years if preliminary development plans are on file with local officials. In Mashpee, planner Fudala said plans are on file for 80 percent of the town's land, many of them submitted days before the one-acre rule was adopted.

Towns are buying up land to preserve it as open space. With $2 million from the state, Mashpee paid $4 million for 250 acres of woodland and salt marsh that would have become a golf course and houses.

The state legislature is considering a bill that would allow Cape towns to tax up to 2 percent on the price of land, with the money going into a local "land bank." According to Doane, a similar law on Nantucket generated $4 million in two years, and such a plan has been approved for Martha's Vineyard.

Federal legislation preserved the Cape Cod National Seashore, a dramatic, 40-mile stretch of beach from Chatham to the Cape's bent fist at Provincetown. But even that park is not immune from change.

The town of Wellfleet once proposed a new garbage dump on its land within the seashore but eventually joined other Cape towns in a plan to truck garbage across the Cape Cod Canal to suburban Massachusetts. Congress has been asked to renew the the seashore's citizens advisory commission charter, allowed to expire by the Reagan administration.

The high cliffs at the seashore, the smooth beaches and the rolling dunes seem to be the last refuge for the solitude of land and sea that Thoreau once found on the Cape.

"It is a precious piece of property," selectman Thomas said of the Cape. "If we don't respect our resources, we will be the losers because, without man, everything would be functioning beautifully."