IN THAILAND, WHERE Richard Castrodale, was a consul (the top job at a two-man post) at Chiang Mai and then Song Khla, his family enjoyed large houses and many servants not to mention the elegant mini-mansions in the gardens for the household spirits.

So it's hardly remarkable that home in Great Falls, Va., where they returned in 1976 after the three-year Thailand tour, seemed a bit cramped. The spirits - who apparently occupy less space than earthly beings - were settled quickly enough. The Castrodales brought back an exquisite house for them, about the size of a large dollhouse, and installed it along the path the children use for the schoolbus.

It took considerably longer - better than a year - to house the other members of the family. The children, of course, take up considerably more space on this home tour than they did before. Rowan is 8, Louisa 7, and Alex, 3. And there were all the things Richard and Diane Castrodale had collected: Wooden temple carvings from Burma; a kite and fabrics from Thailand; a Sven Larsson pine table from Sweden; shells from South Africa and Diane Castrodales native Zambia.

The original house, built in the 30s was mostly concreted block, sturdy, but not too spacious, with rather low cellings. It was well worth its $50,000 cost 10 years ago because of the two acres of land in a pleasant estate section of Fairfax Country. The master bedroom wouldn't hold a king-sized bed and the new handmade Swedish table wouldn't fit.

So the Castrodales took the problem to Richard Ridley, a Dupont Circle architect who has done a number of remodellings. Ridley gave them his customary client-participation test: He asks clients to stick pins in pictures they like on a collage so he can get some idea of their preferences.

More important from the Castrolade's point of view, he made 51 (give or take) different designs for the house before everybody shouted. "That's it." Along the way, of course, they hit on the perfect plan, which cost too much and had to be scaled down.

What the Castrodales got on the 51st try is a large, pleasant dining room not only big enough for the Swedish table and buffet, but a hanging garden of plants. The living room now soars almost three stories high, a step down from the entry way. A study balcony overlooks the living room.

The new master bedroom has a fine compartmented bath, with two washbasins set in a good-looking wooden counter and a two-person-sized walk-in closet. A balcony, reached through double French doors from the bedroom, overlookis the family room.

Across the new hall is a sauna with two benches in case everybody wants to us it at once. A sun-and-moon-viewing round window is rather wasted here, since it is only used coming and going to the sauna, but it does keep what would be a rather narrow passageway from being oppressive. Oh yes, you know how it goes - the sauna was ordered in centimeters, the space was in inches, and so there is this gap, but then it's a handy hole to stash things.

Most exciting is the stairway to the stars, leading up a hatch and thence to a platform (above the sauna) just the right size for the telescope Castrodale's father made for them.

Best - and most amazing - of all, the whole project, which doubled the house's existing 1,632 square feet (plus 160 square feet of deck), cost only about $42,600. The contractor was R.T. Foley of Chevy Chase. Castrodale hired a local man to rebuild the fireplace, installing a Heatilator to make use of the easy availability of wood on the property - a significant factor, they think, in cutting the cost of running the new furnace. The Castrodales did the painting, a lot of caulking and some staining themselves.

One of their great pieces of luck was finding that some of the old beams torn out of the original house were redwood. These were reused as railings and to make a sandbox for Alex. A neighbor told the Castrodales that the house was built using some parts of Harry Truman's presidential inaugural stand among other scavenged materials.

The house has eight inches of insulation in the roof and four in the walls. The exterior is plywood siding, battened. A second (not yet floored) was added atop the old garage, to be finished into another room when needed.

There are, of course, still problems, since whose house is ever finished and perfect? The Castrodales admit they told Ridley to forget the kitchen, since it worked all right - until its back wall became the door to the new family room. The kitchen since has become a busy corridor.

And there are the problems everyone has in settling in. Rowan finds the first-floor bedroom rather noisy when her family gives a party. Alex sometimes gets lonesome for his parents at night and wanders up the two fights from his bedroom

But the Castrodales are coming feel like they're home, and they don't even miss their mansion in Thailand anymore.And so far, there are no requests for remodeling from the spirits in the garden.