If you're thinking of taking a tenant into your home to help cover your mortgage costs, you're part of the latest trend in housing today. One reason that the housing shortage isn't as bad in real life as it is on paper is that thousands of people who can't afford houses or high rents are living in uncounted numbers of in-home apartments.

You might consider taking a tenant if you're a married couple whose children have left home, if you're an older person needing extra money and help with the chores, if you're divorced and unable to handle your mortgage payments alone, if you want to buy a house that you cannot carry without another source of income, or if you just plain want some extra money to spend.

Taking a tenant does change your life. You become responsible for another person's comfort, vulnerable to another person's eccentricities and, depending on the layout of your house, subject to a certain loss of privacy. On the other hand, a tenant can provide companionship, a sense of physical security for people who otherwise would live alone, and an extra pair of hands when help is needed.

And then there's the money. The tenant pays rent and a share of the utility bills. You, as a homeowner, can write off expenses and depreciation on your income-tax return.

For people launching into these uncharted waters, I've come across an enormously practical guide, written by a woman--Doreen Bierbrier--who has taken tenants into her home. It's called "Living With Tenants, How to Happily Share Your House With Renters for Profit and Security" ($7 including postage, from the Housing Connection, Box 5536, Arlington, Va. 22205). The book is no substitute for your own lawyer or accountant when drawing up a lease and settling tax questions. But it has highly sensible answers to many of the business and human-relations problems that anyone encounters when sharing a living space with a stranger.

Many of you may be under the impression that the zoning laws prevent you from taking a tenant. In some cases, that's true. But what is generally zoned out is the so-called "mother-in-law apartment"--separate living quarters with their own entrance, their own electricity meter, their own kitchen and no access to the rest of the house.

In most jurisdictions, it is perfectly legal to share a single-family house with someone (or more than one person) who pays you rent, Bierbrier says. Check this out at the zoning office in city hall or town hall.

There is no simple formula for setting rents. Everything depends on the quality of the accommodations you offer and competing rents in the area. Bierbrier suggests that you investigate other in-home rental units advertised in the newspaper and see how they compare with yours.

"I suggest that you set your rent slightly below the market rather than above," she writes. "Although you will eventually find someone willing to pay a higher rent, you may have a vacancy for a while, and the higher rent will not compensate for the missed rent of a one- or two-month period."

When you take someone into your home, you are free to choose a compatible person. You can reject people who smoke, for example, people too old or too young, people of the opposite sex, or people who like exactly the music you hate most.

There is no need to rent on a first-come, first-served basis. A phone call will screen out some people. A personal visit will screen out others. You are at liberty to tell a prospective tenant that you want to see several people before making up your mind.

No matter how picky you are, your tenant inevitably will have some habits you dislike (as you will have some habits that the tenant dislikes). But you can't clone yourself as a tenant. Choosing a tenant is like getting married, Bierbrier writes. "Before you get married, keep both eyes wide open. After you get married, keep one eye half shut."

Bierbrier strongly advises that you require money up front: the first month's rent, the last month's rent (to ensure that you get at least 30 days' notice before a tenant leaves) and a damage deposit, perhaps amounting to 75 percent of a month's rent, to cover possible damage beyond normal wear and tear. (The laws of your state may limit this up-front deposit.)

She also advises that you ask the tenant to pay a fixed percentage of the utility bills, rather than include that expense in the rent. That way, you don't have to raise the rent every time your utility bills go up.

Becoming a landlord definitely complicates your life. But it's an interesting way of making some money right in your home, Bierbrier concludes.