For many years, a modest, three-bedroom Colonial occupied the corner lot at Gray and Scituate streets in Arlington. Then, in 2000, the owners sold the house and moved away.

North-South Development Co. bought the property for $900,000. After razing the original house, the firm divided the lot to make room for three new houses, all of them larger and more expensive than the original.

''It's tough to find a property with enough space for multiple homes, but when you do, it's a good opportunity," said Michael Collins, a principal of the Arlington-based firm.

Two of the houses were sold this month for $918,000 and $921,000, according to Multiple Listing Service listings. A third is on the market for $939,000.

The conversion of that lot, from one house to three, illustrates what's been happening in one niche of the real estate market as developers and property owners capitalize on what opportunities remain in communities already thought to be built out to capacity.

While much focus has centered on places where developers are able to build trophy homes on sprawling lots that would dwarf a football field, a different scenario exists at the other end of the spectrum.

''In a community that is built out, there is very little if any space left, so someone who's got a large lot is looking for a way to get another house onto it," said Arlington Town Planner Kevin O'Brien. ''Where there was once one home, a person can get in another, or maybe two more."

Residents in some communities are concerned that such development will lead to housing density that is too great for neighborhoods. Some communities, such as Lowell and Winchester, are taking action to head off the trend.

The divisions of lots into smaller parcels is occurring at the same time as a larger trend in which house-lot sizes are on the increase in many of the state's communities, specialists say.

Both dynamics were, to some degree, underscored in a recent analysis of housing data by the Massachusetts Housing Partnership and the MIT Center for Real Estate.

The survey indicated that average lot sizes for newly built houses, condominiums, and apartment buildings have increased to almost an acre in some communities and much larger in others, particularly in outlying areas on the fringes of Greater Boston and farthest away from Route 128.

The study crunched data from property deeds, local assessors, and the Warren Group, a real estate statistics company, to compare lot sizes for houses built before 1999 with those built between 1998 and 2002.

The comparison found that the average lot sizes in Carlisle, for instance, went from 1.69 acres to 3.08 acres per house. Carlisle's town planner, George Mansfield, said the trend toward larger lots is not driven so much by buyer preferences as they are by the need for properties to have enough space for septic systems and private wells for drinking water.

''We have no public water and no public sewer, and, in some areas, a lot of wetlands," Mansfield said. ''All of these become factors when builders determine how much space they'll need to build a house."

Overall, the average lot size for a new house built in the suburbs northwest of Boston is almost an acre, compared to a little more than a half-acre for houses built before 1999.

Clark Ziegler, executive director of the Housing Partnership, said the trend toward larger lots has fueled sprawl development and high housing prices.

''This type of zoning has pulled development toward large lots for single-family homes, which chews up more land and produces more and more sprawl," he said.

But while expansive house lots might be the norm in some places, it's almost unheard of in other communities because buildable parcels are smaller and are much harder to come by.

In these communities, developers and property owners are finding it worthwhile, whenever possible, to reconfigure and divide established property boundaries to fit in more houses.

From a development standpoint, these are well-suited for development. They may seem like postage stamps in size when compared to expansive lots in the outer suburbs, but they have access to municipal sewer and water systems.

What's more, they have frontage on existing streets and also meet minimum-lot-size requirements even after they've been divided. Because of this, they are not subject to complex subdivision requirements and other restrictions that can be imposed by planning boards.

Winchester Planning Board chairman Peter Van Aken said a town's authority is often limited. The town has had 39 lots divided in the past five years.

In some instances, owners are not only tearing down houses to build new ones, they are moving the original houses on their lots to make room for new structures.

''If they meet the requirements, they're legal and there's not much the town can do," Van Aken said.

Town planners say they have seen inquiries from property owners asking about building rights on land upon which they own their home.

''We get them all the time," said Stoneham Planning Board chairman Gus Niewenhous. ''The question is usually, 'Can I divide my property and still comply with zoning on lot size and frontage?' We've had three in the past year that were OK, so it's worthwhile to ask."

Reading's planner, Christopher Reilly, said the town gets about six queries a year.

Lowell, which is in the midst of a development surge downtown, has seen properties divided in neighborhoods like Belvidere and Centralville.

Developers are ''maximizing their land values by doing as much filling in as possible," said George Proakis, chief planner for the city. ''It raises concerns for us about density and design."

In Arlington, Collins said opportunities are becoming increasingly hard to come by, but developers, nonetheless, are motivated by the interest shown by buyers who want new homes in these communities.

The lot at the corner of Scituate and Gray streets measured a little more 19,000 square feet when it was sold. North South Development Co. was able to divide the lot and still comply with Arlington's zoning, which requires house lots of 6,000 square feet.

The houses on Scituate Street, featuring four bedrooms and fireplaces, sold after being on the market for less than 10 days.

''Arlington is a good community with good schools. There's a lot of demand to live there, even if it's already built out," said Collins, whose company is building an 18-unit condo development named Heritage Square on Massachusetts Avenue.

Communities have taken some action. In Lowell last month, the City Council's zoning subcommittee voted to increase minimum lot size requirements from 2,500 square feet to 4,000 square feet to prevent overly dense development.

Winchester planners have drafted a new zoning bylaw that would prohibit a new strategy used by developers and property owners.

Van Aken said cases are starting to turn up in which owners of single-family houses are building second houses, then connecting the two structures with a breezeway.

Under current zoning, this is actually legal in neighborhoods zoned for multifamily residences, but planners say it is merely a way to add another house onto a property.

Winchester Town Meeting will vote tomorrow night on an article that would revise the zoning bylaw to prohibit this strategy.

''It's an end run around our zoning," Van Aken said. ''It's also part of the bigger trend -- people trying to squeeze as much value from their land as they can."

Alexander Reid can be reached at areid@globe.com.

Spreading out

The information below shows the average lot size in acres for houses and condominium and apartment developments in communities north and west of Boston. New construction was built between 1998 and 2002. Existing housing is prior to 1999.

Community	New construction	Existing units
Acton	1.12	0.63
Andover	1.07	0.64
Arlington	0.06	0.13
Ayer	0.48	0.31
Bedford	0.99	0.57
Belmont	0.41	0.19
Billerica	0.63	0.54
Burlington	0.49	0.42
Carlisle	3.08	1.69
Chelmsford	0.48	0.56
Concord	1.30	0.79
Dracut	0.90	0.46
Dunstable	2.64	1.62
Groton	2.15	1.24
Harvard	2.61	1.38
Lawrence	0.07	0.09
Lexington	0.46	0.45
Littleton	1.35	0.84
Lowell	0.27	0.11
Medford	0.14	0.11
Methuen	0.72	0.35
No. Andover	1.08	0.50
No. Reading	1.33	0.65
Pepperell	2.99	0.86
Reading	0.56	0.33
Stoneham	0.35	0.20
Tewksbury	0.42	0.52
Westford	0.96	0.90
Wilmington	0.73	0.54
Winchester	0.31	0.32
Woburn	0.26	0.23

SOURCE: Massachusetts Housing Partnership and MIT Center for Real Estate