Interior designer Theo Hayes had very definite ideas about her latest commission. It was to be stylish but not flashy, tasteful but not prissy.

"I just thought that the Reagans are kind of pretty ordinary people," Hayes said yesterday. "People told me, 'Why don't you put a fireplace in it? Or maybe a stereo system?' But we don't want a doghouse that's a little too frilly, a little too bizarre."

And so Rex Reagan, the King Charles spaniel of 1600 Pennsylvania Ave., will have a white house of his own.

This Friday at 6:15 p.m., no less a personage than Zsa Zsa Gabor will preside at the unveiling of the house at the Vista International Hotel. Her dog Macho -- described by a member of the hotel staff as "a little rat dog" -- will bite the ribbon.

*"It's very colonial, the whole thing," said Hayes. "White clapboard with a cedar shingle roof. I didn't do it feminine or prissy. It's not as if I was doing it for a French poodle or a chihuahua.

"There are draperies of red fabric -- it looks like a Ralph Lauren fabric but it's not. The color scheme of the down dog bed is red, white and blue plaid. By the bed is a night stand with a picture of Lady Marjory Wright and her dog. And on the wall are pictures of Ron and Nancy in acrylic frames.

"Off in the corner, just like in the Oval Office, there's an American flag. Then we have a big silver bowl full of jellybeans and Milk-Bones. I felt like we needed a bit of sparkle. Outside there's a window box in which we'll put red geraniums.

"It's really hard to talk about a dog's house, and this is a hard dog's house to talk about."

Real estate developer Stephen Hayes, the designer's husband and a direct descendant of Rutherford B. Hayes, the 19th president of the United States, installed the front door, raised the roof and laid the attractive parquet floor.

On Sunday, the house is expected to go on display at the annual dog show at the Capital Children's Museum, which commissioned the decoration. Later it will be offered to the Reagans -- although as of late yesterday this was news to Elaine Crispen, Nancy Reagan's press secretary.

"If I was a dog I think I'd like it," said Hayes. "It's very comfortable and not overdone."

The house was originally built 10 years ago by Hayes' father for a Saint Bernard named Patrick. It appears that the First Dog will be able to occupy the house via the canine version of right of eminent domain.

"I think Patrick would be pretty honored to give it up for the Reagans' dog," Hayes ventured. "Maybe he'd be a little jealous because it's got parquet floors, and he didn't have that."

Hayes said this was her first crack at designing a doghouse -- "and probably my last. A dog is a very tough client to work for. To tell you the truth, I don't like dogs. My idea of a pet is a goldfish.