Use the same baby sitter often? Hire a college-student neighbor to cut your lawn during the summer?

Have a regular housekeeper, governess, au pair, handyman, gardener, butler or chauffeur?Congratulations. You are most likely an employer of domestic help, eligible to pay Social Security tax, federal and state unemployment tax, and maybe even withholding tax on the worker's income.

You may not think of yourself as a member of the "Upstairs Downstairs" set, with the kind of resources at your disposal to preside over a household staff.

But the rules say that if you pay anybody as little as $50 or more in any calendar quarter for household services, you may have yourself an employee for whom at least Social Security taxes must be paid.

People who have been through this sort of thing attest that it is an experience full of paperwork hassles and the potential for bureacractic nightmares, dealing with a wide variety of government agencies.

It doesn't take a cynic to acknowledge that a great many household helpers around the country are paid "off the books" in cash. This kind of work is a big and vigorous part of what is known as the underground economy. It is not only cheaper to operate that way, but also a whole lot simpler.

However, household employers who either wittingly or unwittingly fail to follow the letter of the law do so at their peril.

Suppose you hire a regular baby sitter with the understanding that the two of you will "skip the heavy paperwork" and that payments will be made in cash. Things don't work out to your satisfaction, and a few months later you dimiss the baby sitter.

If only just to spite you, the baby sitter could decide to report you to the authorities.

Once they determined that you hadn't complied with the rules, they would very likely start dunning you for payment of both your and the babysitter's share of Social Security taxes owed. Together, that's 15.02 percent of all wages up to $45,000 a year.

Add to the bill interest from the date the payments were due, plus penalties of up to 25 percent of the amount owed and another 25 percent for failure to file the required returns.

"Furthermore, you could also be subject to penalties for negligence," observes the accounting firm of Spicer & Oppenheim.

Beyond Social Security, federal unemployment taxes must be withheld for anyone whose pay exceeds $1,000 a year, while state rules vary.

Earnings for domestic work performed in a private residence are not subject to withholding for income tax purposes, unless both the employee and you want to handle it that way.

This is a fine distinction in an area of the tax rules that is full of fine distinctions.

In the daily busyness of life, it is easy to let such matters slide. It also can be very costly.