FROM THE outside you might think the George Mulligan house started as a mill. If it had wheels on the side - its most impressive view - you'd surely bring it your corn to grind.

Inside, you would be sure it was born as a mill. Look at those 1,400-pound red oak beams. Actually, its construction is that of a northeastern mid-19th-century barn with mortised and tendoned joints, post and beam construction.

"We first saw a house like this one when we were visiting in Vermont," said Mulligan. "When we walked in, we said, 'This is for us.' We had been through so many house plans, trying to find one to suit us: 15 schematics and three sets of, working drawings.

"We knew we wanted a functional, friendly house. David Howard in Alstead, N.H., engineered the house, and cut all the timber. I doubt we could get wood like this in our area. He sent down a crew of five people to put the basic post and beam structure together. You should have seen their big tool box - the size of a coffin, full of wonderful old-time tools.

"The house, after the structure was complete, was really designed as we finished it up. With this post and beam construction, you have freedom to put windows and walls pretty much where you please."

Mulligan was used to the ways of craftsmen, from his years as a builder (he started out with Edward Bennett, worked as a developer, and now is a speculative builder).

"But I was amazed at how interested they were in this house and what a difference it made in the quality of work they were willing to put into it. They'd even bring their wives to see it. Harry Harne, the plasterer, gave us rustic plaster walls that were just right to contrast to the handworked wood. Joe Pagliaro, the stone mason, found us the exact size of stone we needed for the front door stoop - a design detail we hadn't worked out. The electricians and plumbers had a complicated job to make everything fit. Because there is so much structure exposed in the house, there're very few hiding places."

Mulligan recently built a second house like his on 2 1/2 acres in Potomac, and sold it right away for $250,000.

Mulligan's own house if not immense - though neither is it cramped, at 3,200 square feet, but the volume of space makes it seem much larger.

A fanlight over the door brings the sun in, blessing the house with a bright passage. At one time, such windows were a standard feature of self-respecting houses, a Palladian influence lasting long after Palladio's less-useful fancies died.

George and Barbara Crawford Mulligan not only have a fanlight above their front door, they also have an incised gilded sun (not to mention a carved pineapple, the hospitality symbol older than the welcome mat).

The house has the right to claim the sun as its emblem. Entering the center hall, you are warmed by the 4-by-4-foot skylight, beaming down three stories. Flying bridges span the second and third stories, linked by the staircase and forming a dramatic sculpture in the three-story high hall.

The house also admits the sun through tall, wide windows. When the house was under construction, its clear span construction made it possible for the Mulligans to wander through and say, "We want a window here so we can see that tree. We want to wall there so we won't have to see that house."

A ssecond fanlight arches over the double French doors to the rear. The house is built in classic center hall style, with back and front doors lined up to blow a sweep of air through the hall in the summer. In old Virginia houses, one door faced the river, the other the road, but nowadays not everyone has a river at the back door. The Mulligans do have a stream.

There is a big fireplace in the living room, facing the bookcase wall and the playpen sofa. More practically, there's a Jotul wood heater in the three-story hall.

Most of the house's spaces flow into each other. The dining room is separated from the kitchen only by a counter. Barbara Mulligan's third-floor aerie is open to the bridges so she can keep an eye on what's going on below. One mistake the Mulligans admit: Louvered doors for every room are just too open for privacy.

There are closed spaces.The master bedroom and lavish dressing room are separated from the children's rooms by the second-floor bridge. The guest room and bath are across the third-story bridge from Barbara Mulligan's study.

Mulligan has a pleasant office for his building business with an adjacent outside door. A coat closet, bath and a first-floor laundry room are convenient, but tucked in off the beaten path. (Mrs. Mulligan doesn't need a laundry chute; she just drops clothes down the three-story hall.) George, 9 1/2, and Lisa, 8, have their playroom with television in the basement.

The children find the bridges and open stairs as good as anybody's jungle gym. They race up and down and hang over the bannister as if they were in the circus. The children, however, don't think the house is all perfect. Somebody goofed and put plastic tile on the basement floor, instead of leaving the concrete bare for skating.