In the mid-1960s, when suburban subdivision development was taking off on a high-flying course that extended into the early 1970s, the small Montgomery County village of Potomac began to lend its name to a fast-spreading area of homes west of Seven Locks Road.

Today, "Potomac" describes a vast swath of the Western county, including a wooded, rolling area west and north of the original horse country village where most of the lots are more than two acre and sewage lines still do not reach.

Much of the increased pace of development of homes ranging from $150,000 to $600,000 is taking place on lots that still must be served with septic tanks. Prices for these building sites start at $40,000. This area is west of Piney Meetinghouse Road to Travilah, Riverwood and out near Seneca and east near Falls Road.

Dozens of small-volume builders have been developing large-lot Potomac homes for years, as commuters on River Road know. However, one developer estimated cautiously that home building and lot development in this unsewered rural area has increased 100 per cent in one year. Gullies, valleys and hills are crisscrossed by streams that include Watts Branch, Piney Branch, Sandy Branch and Greenbriar Branch.

One of the new homes in the spring spotlight in Potomac is the latest creation of builder Leo Patrick Cullinane Jr. Located off Piney Meetinghouse on Piney Glen Lane, this Cullinane-designed mansion has a six-pillared front portico leading to a 20-foot-high great hall -- with 18-by-26 feet of marble floor -- in the style of Thomas Jefferson's Monticello. The surrounding frieze done by a local sculptor was copied from Jefferson. And the great hall entry is sunk inside two easy steps that lead to a striking drawing room extending to the back of the house.

But that's only the center of this 8,500-square-foot brick dwelling done in Palladian architectural style.

It goes on from there with a formal dining room opened to the great hall, a broad winding oak staircase hall that was inspired by a house Cullinane visited in England. The kitchen is super-ample, even with a double refrigerator with paneling to match the cupboard. The family room is authentic antique brick taken from Charles Center in Baltimore. None of the seven fireplaces shows any firebrick, which Cullinane considers repugnant: He covers firebrick with brick.

Upstairs are five bedrooms, with the master suite having the dimensions of a generous apartment. Bathing and storage facilities are large.

The elegant, colonial touch of the fortyish builder was developed over the years in working with his builder father, now 70 and recovering from a stroke. They have built earlier homes in Burningtree, where their intricate brickwork and colonial styling was designed to give the impression of an older home.

The exterior of the younger Cullinane's new $600,000 house -- whose eventual owner will no doubt sense its potential for grand entertaining in a setting of quiet American elegance -- is distinguished by Flemish bond brickwork, bricked steps with custom rounded edges and generous use of dentils, archways, fanlights and four chimneys capped with stone.

Even the main central supports in the basement, which will be finished to suit the taste of the buyer, are massive stone brick arches. The basement has 3,200 square feet. The sleek slate roof has eagle forms as snow-stoppers. And the front windows are trimmed with Chippendale rails.

The Cullinane house, about which other builders comment as athletes would discuss O. J. Simpson, Jack Nicklaus or Dr. J., faces another large sold and occupied Cullinane dwelling across the street.

Meanwhile, other builders are doing their own number in Potomac.

Around the corner from Cullinane is a newly completed "spec" house that was built by the John Matthews-James Schwartz partnership. Schwartz said the house was sold for $290,000. And an architect is completing a new house for his own use that is as elegant as Cullinane's latest.

Schwartz-Matthews, who have been building and developing together for 14 years, have started a small Potomac subdivision of 24 lots at Saddle Ridge; three of the five houses now being constructed are sold.

"It's a rough, hilly property that colonial builders avoided, but it's fine for our contemporaries," said Schwartz, who is afraid to set prices for his houses prematurely because of escalating costs of materials. At the moment, the homes are tentatively priced at upwards of $195,000. A few years back this same team built other contemporaries at Timberwood of Potomac off Falls Road in the $70,000s.

Further out River Road, almost to Travilah Road, developer-builder Clarence W. (Bud) Gosnell, whose own family home with stables is on Piney Glen Lane, has been doing his style of early American custom building on two-acre lots that were subdivided from a portion of the original Drew Pearson farm, which still exists on the river side of River Road.

Gosnell's Beallmounnt $200,000 houses have four to six bedrooms, multi-baths and finishing details such as open stairwells, with spindles and railings done by his veteran carpenters. Most of his current construction is sold out and is expected to be completed later this year but the house he uses as a model is still on the market.

Another Potomac specialist, Croswell & Baker, has been creating large homes on large lots at Riverwood, further south River Road. The entrance to this community is accented by a gazebo on a pond and most of the houses are built on hillsides. Currently, this firm is completing French chatueau-styled houses, priced at more than $250,000, in River Woo and also in River Oaks, which is closer to Potomac Village. The two-story family rooms have hand-laid stone fireplaces.

Cowan & Hodgkin is another Potomac building team with three houses under construction at Tara, north of River Road and west of Potomac Village. At Tara, where Bruce Winston and Robert Blitz, have been developing two-plus-acre lots on a heavily wooded and ravined tract, individual ground costs now are reported near $70,000.

"We have delivered two of our houses that are in the $260,000 range," said Wes Hodgkin, who added that the currently favorable mortgage money market seems to have increased buyer interest in expensive Potomac homes. "Most of the buyers have sold homes on which they had considerable gain," he added.

The desire for homes in the area around "the village" is described by veteran builder Steven Kokes --currently finishing a $500,000 custom home for a buyer on River Road -- as "Potomacitis." He said the desire for large houses is usually prompted by a need to entertain large numbers of friends more than occasionally and to have a dwelling whose being bespeaks stature. Opposite Congressional Country Club, Kokes has a "spec" house that is still available to a buyer with $480,000.

Kokes described the Potomac market as "unpredictable, because it can run hot for three or four months and then it goes cold. It's got to be related to the economy. Now it's good."

S & J. Builders, D. Jay Hyman, Brendan O'Neill and others are among those building houses here and there in Potomac, where individual lots have been developed over the years from the Beall farm, the Norton farm and others.

The big Plummer, Heurich and Pearson farms will probably be the sites of houses as the market for large homes with acreage expands.

Potomac seems certain to remain a prestigious residential area as long as affluency enables home owners and prospective buyers to handle down payments, monthly mortgage payments, taxes and utility bills. CAPTION: Picture 1, This Palladian-style mansion on Piney Glen Lane near Potomac was built by Leon Patrick Cullinane. On the market for $600,000, the house has, 8,500 square feet, a six-pillared front portico, seven fireplaces and a family room lined in antique brick from the Charles Center area of Baltimore. A winding oak staircase, inspired by a house Cullinane saw in England, leads to the four second-floor bedrooms and master suite; Picture 2, The marble "great hall" entryway of Leo Patrick Cullinane's latest Potomac house was fashioned after that of Thomas Jefferson's Monticello. It's Monticello-style frieze was copied by a local sculptor. Sunken hall opens on dining room and on drawing room that extends to the home's rear, by Doug Chevaller --