Tax giant H&R Block Inc. has a lock on one of life's two certainties. Now it is going after the other.

Two months ago, the Kansas City firm launched a new service --online and in-store software packages designed to help everyday people to write their own wills, trusts and estate plans at home without help from lawyers.

Selling do-it-yourself legal kits isn't a new idea.

An estimated seven in 10 Americans have no formal wills other than to follow individual state formulas for dividing property or providing for heirs. Of those with formal plans, the average age of the wills, when opened for final reading, is about 20 years old and potentially outdated by changes in estate planning law or family circumstances.

Block and others see potential business opportunities in the void.

Block has been peddling versions of its new WillPower and Home and Business Attorney through its tax offices and other outlets since at least 1996. Some competitors, notably Nolo.com in Berkeley, Calif., publisher of Quicken estate planning and legal documents, have been providing self-help legal information and forms to nonprofessionals since the early 1970s. Newer players, such as LegalZoom.com, a Los Angeles online service co-founded by O.J. Simpson attorney Robert Shapiro, even fill out the forms.

Competitors don't provide many precise sales figures. Nolo has disclosed that its Quicken WillMaker Plus sales increased nearly 33 percent during 2006. LegalZoom says that it has served 500,000 people since its opening in 2000, and that sales have been growing 50 to 75 percent a year. We The People, a storefront franchise that helps people fill out legal forms, has grown to more than 100 locations, including one in the Kansas City area, from just 25 seven years ago, according to its Web site.

These providers offer information or educational material, but they stop short of advising buyers in order to avoid what in many places would amount to practicing law illegally. They also cost a lot less than traditional legal help.

Basic, commercially available do-it-yourself wills and estate planning documents in most states run between $20 and $120. Hiring a lawyer to do the same work appears to run between $700 and $1,500 in many places, but can vary widely.

Which choice is right for you depends on your circumstances, authorities say.

"Those little kits could work if you have the training, education and expertise to use them properly," said Lee Davis, a Johnson City, Tenn., lawyer and president of the National Association of Estate Planners & Councils.

However, there also is lot of potential for someone insufficiently versed in legal matters to create major problems for heirs, for example by leaving out something important or using incorrect or ambiguous language, Davis said.

Hiring a lawyer also becomes an increasingly better idea if your estate becomes bigger or more complicated, or if your or your heirs' circumstances are changed by death, divorce, remarriage or some similar event, Davis said.

"Kits just don't cover all those conditions," he said.

Spending less than $20 for, say, Block's basic will and trust software or less than $40 for the expanded package, with software that helps fill out many other legal documents a household head or small-business owner might need, can be economical even if you need a lawyer's help with the final product, countered Jason Bass, a digital products manager at Block.

"We aren't a substitute for an attorney," Bass said, "but using the software to pull together and organize your records ahead of time will produce a significant savings in legal fees."

That is because organizing the documents is cheaper than paying your attorney's staff to do it, he said.

Block's new software products, linked with Acendi Interactive, a San Francisco developer of RocketLawyer.com and other legal self-help technologies, bridge a gap between traditional forms that consumers fill out themselves and new services like LegalZoom, in which the service fills out forms based on questionnaires the clients complete.

But, again, "we can't give advice and we don't," said Mike Turner, a LegalZoom spokesman.

Wills, trusts and other estate-planning documents generally are private documents that in all 50 states can be drawn up without a lawyer. Virtually anyone 18 or older can make a will.

But some rules vary. Kansas law doesn't recognize so-called holographic, or handwritten, wills, while Missouri law doesn't say whether it does. Individual states also have slightly different rules about witnessing the documents and notarizing some of them.

Many authorities broadly suggest using a lawyer, rather than online software or a legal kit, if your estate exceeds the $2 million exemption, above which federal estate taxes kick in. Online software or a kit probably is sufficient for simpler or smaller estates, particularly if most of the property being distributed is personal property.

But that isn't always true. Wills and trusts are so intrinsically private that "we really don't have a good idea what sizes are involved," said Turner.

On the flip side, even simple estate planning could get complicated. The current $2 million exemption from estate taxes, for example, is scheduled to jump to $3.5 million next year, disappear entirely in 2010, then drop back to $1 million after that.