Building your own house generally means having a contractor custom-build the kind of a house that you want built. And that was the procedure followed by Leonard and Elsie Thomas, who now are living in a new, Tudor-style house in the city of Fairfax. Here's how they did it.

The story begins about 12 years ago when the Thomases bought an existing house in the Little River Hills section of Fairfax City. They like the house but, not unlike many other homeowners, had a dream of building their own to their own tastes and needs.

Well they learned about an available lot, slightly more than an acre, adjacent to the house they occupied. They liked the lot and bought it 11 years ago for about $6,000. That was the first step.

"I farmed part of the land and enjoyed it until the autumn of 1975, when we decided to build our own dream house or sell the land," recalled Thomas. "Building won out and we set about finding a house plan to suit our needs and desires."

After looking at many plans in magazines and books, "we found our perfect plan in a House of the Week plan in The Washington Post," said Thomas who happens of be the makeup director of this newspaper. They sent $1 for the baby blueprint of Plan R-101 and liked what they saw even more. "So we paid $70 for eight copies of the actual blueprints of the house designed by Herman York of York & Schenke." (York, incidentally, has designed hundreds of single-family house in the past 25 years and has acquired a national reputation.)

The next step was getting bids from builders. "The builder we chose," according to Thomas, "was one of two in the same money range. The deciding factor was the enthusiasm of Vernon Brake and Dale Keesey of Gaithersburg. It almost matched our own."

Next, the Thomases tackled financing. They shopped the market and wound up getting a construction loan and a permanent financing commitment from Arlington Fairfax Savings & Loan Association, which had the delete mortgage on the house in which the Thomases had lived for 10 years. "The lender was very fair and beat the other in terms of interest rate and also stipulated no penalty if we are able to pay down the new mortgage in the early years," added Thomas.

With the plans in hand, the builder selected and the financing arranged, ground was broken July 22, 1976. "That was the beginning of six months of pleasure, grief, enthusiasm, disappointment and a great deal of leg work. But in the final analysis we achieved a real sense of accomplishment and enjoyment," Thomas said.

On Feb. 1, 1977, the Thomases moved into their almost completed house that they had watched go up day by day. In addition to the $6,000 lot, which turned out to be a bit of a bargain, the house cost about $74,000. It has 2,300 square feet of living area plus 350 feet in the two-car garage.

In particular, the Thomases like the 10-foot-high ceiling in the living room of their brick and stucco two-story dwelling, which has three bedrooms, two and one-half baths, living room, dining room and kitchen. They plan to do a recreation room themselves in part of the partial basement of the fully electric house, which has two heat pumps and zoned heating and air conditioning.

For energy conservation, the house has 8 inches of blanket insulation in the attic and 3 1/2 inches in the walls. The windows are double-paned.

During construction, the Thomases added and paid extra for a third fire-place (in the basement where the rec room will be), a walk-in storage area above the garage with entry through an adjacent bedroom and an outside entrance to the basement.

That outside entrance has caused one of the few real problems for the new owners, who generally are highly satisfied. "This is a soundly built house and the only flaw we've encountered so far is the provision for drainage to the outside basement steps. The dry-well installed by the builder is inadequate in times of heavy rain and we get some water in the basement via the door at the bottom of the unprotected stairwell.

"However, we are confident that the builders will do their utmost to correct it," said Thomas.

In summary, the Thomases and their two daughters like the privacy of their lot, the living-room which stands off by itself, the sense of spaciousness in the foyer with a cathedral ceiling and balcony above it.

Would they do it again? "Absolutely," Thomas said. "It was both fun and work. We selected our own lighting fixtures and carpet. Adding extras cost us about $5,000 but it was all spelled out in the contract. Middle-line appliances and such are usually specified and we upgraded as most people do."

What would you do differently? Not much. "Probably a full basement," said Thomas, who also advised prospective dream house "builders" to count on spending nearly 10 per cent more than the original contract to take care of modest extras and upgradings.

Thomas obviously likes his house because it provides a real sense of personal taste and contribution plus a semi-vicarious feeling of having built it.