Why are Californians in a frenzy over their property taxes? The answer is no mystery to Los Angeles County homeowners, some of whom have had these experiences within the last two weeks:

The owners of a ranch-style home in the scenic Palos Verdes peninsula in southwestern Los Angeles County found that their property taxes had increased from $2,100 in 1977 to $5,500 in 1978. They bought the home for $125,000 two years ago, but comparable homes in the area are now selling for $175,000 and more.

In West Hollywood, a home purchased in 1977 for $120,000 had annual taxes then of $3,700. A new assessment raised taxes to $7,000.

In Woodland Hills, at the West end of the middle-class San Fernando Valley, a house bought last summer carried property taxes of $1,900. The new tax bill, unless Proposition 13 is approved by the voters on Tuesday, will be $4,100.

These increases in assessments explain, far better than any "sendmessage" analysis, why Californians in general and Los Angeles residents in particular are expected to give overwhelming approval to Proposition 13, which would limit property taxes to 1 percent of assessed valuation.

All of the above examples are among the 500,000 homes - about a third of the county's single-family residences - that were reassessed this year.

Under California law the assessor must determine the market value of a home based on comparable sales, if any. State law then directs that homes must be assesed if any. State law then directs that homes must be assessed at 25 percent of market value.