Take a drive down Arlington's Rock Spring Road, a tree-lined street with an eclectic mix of old houses on large, half-acre to acre lots.

At one end of Rock Spring stands a large old farmhouse, primly painted in blue and white; at the other, a mountain chalet and a log cabin hide among towering trees. In between are several bungalows and cedar-shingled farmhouses, all surrounded by flowering dogwoods, azaleas, hollies, maples and oaks, many more than 100 years old.

There is shade almost everywhere -- except smack in the middle of the block where the sun beams brightly on four new luxury homes. The sunshine is as startling as the houses, so large they tower over the adjacent homes and so clean and crisp-looking, they seem jarring next to the old farmhouses.

In this small cul-de-sac, the country-road charm of Rock Spring abruptly has given way to the harsh reality of today's real estate market: Land in close-in, established suburbs has become so attractive and valuable that builders are rushing to fill any vacant lot they can find.

Builders are erecting houses on land once considered too steep, too damp or even too small to be built on. They're tearing down old small houses and replacing them with large new ones. They're buying up old family properties from aging residents or children who have inherited them and dividing the land into several lots to create whole new subdivisions.

In the process, the builders are creating large, multilevel houses that fill every square inch of land allowed by local zoning ordinances.

The result is the creation of neighborhoods with sharply split personalities. It's happening not just on Rock Spring Road but in almost every well-established neighborhood in the Washington area -- and with increasing frequency.

"It's been more visible lately," said Gary Fuller, director of planning for Falls Church, where many humble ramblers and bungalows are being razed to make way for town house developments, large brick colonials or contemporary houses.

"I've been doing zoning for 16 years and this is more extensive now than I've ever seen it and I expect more in the future," said Frank De\Lange, zoning supervisor for Montgomery County.

In parts of Bethesda, Chevy Chase and Potomac, the trend is particularly evident, De\Lange said. "People are going to the max, building as close to the property line, putting the house as high as they can and filling out the lot as much as they can."

The houses all fall within the local zoning requirements, De\Lange said. But, he acknowledged, "next to homes developed earlier when people didn't spend as much money, they stick out like a sore thumb.

"This is not a phenomenon restricted to just this area," he said. "It's happening in New Orleans, California and lots of other places in the U.S. where the value of land has gone way up and there's not a whole lot of development land available."

In the building business, this phenomenon is called "infill" and wher\ever it's happening, it's stirring up controversy.

Part of the contentiousness stems from the size of the new homes; part, from the loss of large trees and open spaces neighboring residents have long considered to be their own private parks.

And part of the controversy has to do with the very nature of development, said Debbie Rosenstein of Ro\senstein Research Associates Inc., a McLean market research firm for developers and builders.

"No matter if you're building at the end of the earth, the current residents don't want development because it will intrude on the rural' nature of the neighborhood," she said.

Infill -- and the disputes and dismay that come with it -- are certain to continue for the simple reason that close-in locations are in demand. Home buyers want to live in communities where the daily commute to work is short and where there are existing roads, parks, shops and a well-established school system.

But "the realities of the market are that people don't want small old houses built years ago," Rosenstein said. "They want large houses, like they could buy in the far-out suburbs: houses with two-story foyers, family rooms on the first floor, a large expanse of windows," even if it means looking into the side of the neighbor's house, "walk-in master bedroom closets and huge bathrooms."

These requirements demand big houses -- and big prices as well. The new houses on Rock Spring Road are some of the most expensive built on infill lots, selling for around $800,000. But even the most modest of town houses on infill lots are commanding prices of about $200,000.

"That's the nature of the land costs and the trade-off for living close in," said Wayne Van Ostrand, director of marketing for Ryan Homes, which has just started developing a town house enclave in North Arlington.

A three-level town house in the 31-unit development at Dorsey Woods starts at $240,000; a similar product farther out, in Fredericksburg, for example, "costs $100,000 less," Van Ostrand estimated.

Traditionally, it's been the small, local and usually family-held companies that have been developing the infill sites, building either one or two houses at a time or as many as a dozen. But as the demand for infill sites grows, these lots also are attracting the big production-line builders as well, companies such as Ryan, Ry\land Homes and Pulte Homes.

"We're doing more than before," said Van Ostrand of Ryan, part of NVR, the area's largest home builder. "It's not part of a plan but just happening because there are opportunities everywhere."

If there's enough land to develop at least 20 town homes and "if we think the price is right, we'll do it," he said.

For Ryan, it's the opportunity to attract a type of buyer the company wouldn't find in its subdivisions in the farther-out suburbs.

"It's a built-in market," said Ralph DeSena of R.A.M. Investing Ltd., a Rockville building firm that has developed infill projects in Montgomery County. "People like where they live and want to stay if they can find a house that's newer with modern conveniences."

That's the chief advantage of infill, DeSena said. Yet, he added, "there's one potential disadvantage: an awful lot of people are watching every move you make and you have to make sure everything goes right."

Scrutiny certainly was the case on Rock Spring where the Choate Group bought about two acres of land two years ago to build six houses; four have been completed and construction has started on the remaining two.

"I would say this is one of the more controversial" infill projects in Arlington, said Robert Brosnan, acting director of the county's Department of Community Planning, Housing and Development.

Like many lots on Rock Spring, the land had been held by a single family for decades. The Burns family bought it in 1912 when it was a cornfield; Walter W. Burns had lived there since 1953 in a small brick and frame house. At the age of 73, he decided to sell; "My wife wanted to live in a different place," he said.

The buyer: Todd Hitt, an Arlington native and son of Russell Hitt who owns Hitt Construction. Preferring to build houses, Hitt started his own firm, Choate, nearly three years ago.

Hitt, 29, was well-known in the neighborhood because just a half mile away he had just completed an even more criticized project.

In that project, known as Brandywine, three small houses were torn down to make way for 11 huge brick colonials, half built by Hitt, which the neighbors have dubbed "McMansions" for their size and box-like similarities.

Today, Hitt has acknowledged that the builders "could have taken another tack and built fewer houses" at Brandywine" to save more trees. However, he said, "the economics of the land acquisition dictated some of the decisions."

Hitt makes no apology for the kinds of houses he built at Brandywine.

"What existed in the neighborhood were small brick colonials built in the '40s and '50s," he said. "Most of those homes are small and not marketable to people coming in to buy new homes. "People are looking for four bedrooms and three baths upstairs and a living room, dining room, breakfast nook, large kitchen, family room and library downstairs. You can't accommodate all those things in a much smaller house.

"That's the reality of the marketplace and the first thing you have to do when you build a house is be loyal to the marketplace; you have to build what can and will sell."

Even so, Hitt decided to do things slightly differently with the subdivision on Rock Spring, which he named Whitehall. Large houses were still a market necessity, but they didn't need to be "the standard brick boxes" found on many infill lots, he said. Hitt said he tried to design the houses to blend in with the neighborhood, applying many features of the existing homes -- cedar shingles, stone chimneys and front porches -- to his new ones.

Early in the project, Hitt met with the neighbors, who were disturbed by the loss of the 50-foot-tall trees as well as by the huge amount of dirt that would be needed to level out the sloping land.

With all truckloads of dirt Hitt brought in, "it was infill squared," said Fred Hannett, whose log cabin sits right next to the Whitehall subdivision and was the most affected by the development. "We used to have a gradual slope in the side yard; now there's a 10-feet-steep angle slope."

The Hannetts appealed to every possible government agency. "We tried to deal with county board members, the Army Corps of Engineers, Virginia's Department of Environmental Quality," Fred Hannett said. "A lot of people felt sorry for us, but no one wanted to do anything because the project met the letter of every {zoning} law."

Anger with Hitt has died down somewhat as residents acknowledge that if it weren't Hitt, another developer would be building on the lot.

But Rock Spring residents remain critical of county officials for their failure to curb the size of new developments -- both the number of houses and the size of the homes.

As JoAnne Hannett said, "I think the county blew it; it allowed {Hitt} to take down every tree and truck in dirt and create an unnatural setting for this street."

County officials said, however, that they had little choice as long as Hitt's project met every zoning and construction ordinance.

Periodically, when disputed infill projects come up, the county has thought about revamping the rules to limit the size and height of the new houses. But repeatedly, modifications have been ruled out.

"I don't see any easy way for the county to make any changes," county board member Mary Margaret Whipple said. "There are property rights involved. And if you change zoning rules to limit size you could render many existing homes nonconforming' with the county code. That has all sorts of implications with financing and could prevent additions."

Among other things, existing residents could find themselves unable to add a porch, deck or addition because they may violate the new zoning rules.

"Over time, as trees and landscaping grow up and as a large number of older houses in Arlington are added on to -- existing homes are getting larger, too -- these {new} houses will seem less obtrusive and incompatible with other houses in the neighborhood," Whipple said.

Despite the inevitable neighborhood uproar that infill development brings, it is, ironically, the neighbors who end up moving in. Michael P. Walsh's family, for instance, just moved into one of the Rock Spring houses last week, moving from a house only three blocks away.

"We wanted a new home because the one we were living in started to need routine maintenance that I didn't have time to do," Walsh said, standing amid boxes of books in his basement office.

"We could have gotten a new house farther out -- but I couldn't stand the idea of a long commute. And we wanted to stay in the same neighborhood because my 16-year-old son's friends are here."

Slowly but surely, the neighbors are getting used to the new houses. The Hannetts have spent thousands of dollars planting pines and other fast-growing trees to regain the privacy they lost when the houses were built. "We will get used to it ultimately," Fred Hannett said.

Still, the Walsh family shouldn't expect a welcome wagon any time soon. As one neighbor said, "I'm sure they're nice, but somehow I just don't feel like baking them a batch of chocolate chip cookies." CAPTION: At the Whitehall project on Rock Spring Road in Arlington, builder Todd Hitt said he has tried to design houses that blend in with the neighborhood. CAPTION: Modern houses are neighbors to bungalows and cedar-shingled farmhouses.