The advertisement was only three lines long, and Sarah Martinelli almost missed it.

"By owner, 2 br, 2 ba, fam rm, sep din, fplc, many xtrs. View lot. Landscaping. Nr. frwys & Rec. Pet OK. $64,000."

One telephone call and the 40-year-old divorced lab technician and mother of a 16-year-old son was convinced her long search for a home was over. She ran to her car, sped to the address -- and found herself in a mobile home park.

"I was mad as hell," she recalled. "Nobody had mentioned the words 'house trailer." It was a dirty trick and somebody was going to get an earful.

"Well, the minute I drove inside, I got the surprise of my life. The park was fantastic. The mobile home was beautiful. And best of all, I could afford it. The only problem was the park wouldn't accept children."

With the median price of an existing single-family home in excess of $109,000, mortgage interest rates of more than 15 percent in Los Angeles and a rental vacancy rate of less than 1 percent, the housing crunch is on in earnest.

And people who once turned up their noses at mobiles homes as housing for the poverty-stricken and mobile home parks as tourist courts for retirees are beginning to have a change of heart.

To begin with, the name mobile home is a misnomer. These days, size alone (in California, nearly 80 percent are at least 24 feet wide) makes the homes as mobile as a football fan seated on the 50-yard line at the opening of the Super Bowl.

In fact, industry associations now refer to their product either as manufactured housing or immobile homes even though the finished unit still sits on a vehicle undercarriage with axles and sometimes tires.

Secondly, today's mobile homes are downright luxurious, containing all the features of a conventional home and such extras as fireplaces, jacuzzis, trash compactors and two-car garages.

Though California mobile homes carry price tags of up to $80,000 (higher than anywhere else in the nation), the cost generally figures out to about half that of a comparable conventional home in a middle-income neighborhood.

Finally, mobile home parks are becoming more posh. Streets are paved and named, and parks have rules governing the appearance of mobile homes. Often there are such amenities as swimming pools, tennis courts, a clubhouse with meeting rooms, facilities for everything from bridge clubs and billiard games to wedding receptions and even golf courses.

The California Department of Real Estate reports that in 1980, the state had a total of 5,700 mobile home parks containing more than 415,000 mobile home spaces and an estimated one million people.

About 20,000 new mobile homes were sold in the state last year, according to the California Manufactured Housing Association. Sales would have been higher industry spokesmen contend, if parks hadn't run out of space and there hadn't been so many restrictive ordinances governing the siting of mobiles.

That, however, is changing.

A new state law that went into effect Jan. 1 permits the development of mobile home subdivisions and condominiums. This means that instead of renting space in a mobile home park for up to $500 a month -- the ones one the beach or with an ocean view -- owners may purchase their own lots in appropriately zoned areas. In return, the homes must be placed on permanent foundations.

Another new law, scheduled to go into effect July 1, would allow mobile homes built to federal standards enacted in 1976 to be sited on individual residential lots if the mobile home is deeemed compatible with the rest of the housing in the neighborhood.

The only solace local governments have -- and it's not much since Proposition 13 -- is a third new state law mandating that all mobile homes regardless of age or size pay county real estate taxes instead of state vehicle taxes. This year, the purchaser of a $30,000 mobile home will pay $282 in property tax instead of $420 vehicle tax.