It’s the downside of the housing boom: Many homeowners are significantly underinsured.

Americans have been pouring money into their homes in recent years, adding everything from marble bathrooms to fancy back-yard decks: Last year alone, spending on improvements like these hit an estimated $155 billion, up 27 percent from two years earlier.

At the same time, the global boom in prices for lumber, copper piping and other necessities as well as rising labor costs have pushed up building costs by 7 percent a year since 2001.

As a result, people who haven’t updated their insurance policies in a few years may be underestimating what it would cost to rebuild their homes, particularly in high-priced markets.

According to a survey by Marshall & Swift/Boeckh, a firm that supplies building-cost data to insurers, 58 percent of houses are under-valued for insurance purposes. Of those, the average homeowner has enough insurance to rebuild only about 80 percent of his or her house, according to the survey.

Meantime, many insurers have been quietly cutting back what their policies cover. Allstate Corp. stopped covering earthquake damage in most states. Several insurers, including State Farm Insurance Companies and Farmers Insurance Group, a unit of Zurich Financial Services, have stopped covering wind damage in some coastal areas that have been threatened by hurricanes.

One of the biggest shifts by insurers in recent years has been the virtual disappearance of “guaranteed replacement cost” coverage, which promised to rebuild a home exactly the way it was, no matter the cost.

Now, most standard policies provide only “extended replacement cost,” which offers up to 20 percent or so more than the face value of the policy if extraordinary events push up rebuilding costs.

Insurance experts say many homeowners haven’t grasped this shift, and may be woefully underinsured as a result.

Home insurance exists to help owners repair or rebuild a home, and replace furniture, clothing and other personal property, in the wake of a fire, burglary or other calamity covered by the policy.

Some risks, such as flooding or acts of war, are routinely excluded. (Policies also include liability coverage to protect against lawsuits resulting from incidents around a home, such as your dog biting the cable guy).

For insurance purposes, the value of a house is based mostly on the rebuilding costs in a particular area, not on its market value. The policy isn’t meant to include the value of the land underneath, which is why some homes, especially in desirable neighborhoods where land is pricey, need less insurance than the amount they would fetch in a resale.

Homeowners need to pay close attention to what’s in their policies because insurers regularly fiddle with the coverage. Insurers notify policyholders about coverage changes in the annual policy-renewal statement, but many homeowners don’t bother to read it, focusing only on the premium.

“Take the new policy and the old one, and put them side by side and if you have questions, contact the company,” said Don Griffin of the Property and Casualty Insurers Association of America. It’s also a good idea to have a face-to-face meeting at least once a year with your insurance agent or broker, at renewal time.

Some consumer advocates have blamed insurers themselves for homeowners being underinsured, saying their agents lowballed replacement costs to keep premiums competitive. Over the past few years, spurred in part by lawsuits, insurers have taken steps to improve their estimates, industry officials say.

That is what Ross Quigley of Mount Lemmon, Ariz., says happened to him. In 2003, a mountain cabin he owns in this vacation community near Tucson was destroyed by wildfire, along with hundreds of other homes.

The retired real-estate broker had a $160,000 policy, but he learned that rebuilding the 1912 structure was going to cost $500,000 because of its unusual features and remote location. Quigley says his insurer had “told me not to worry, because I had ‘replacement value,’” a provision in the policy that stipulates he would receive the value of his home without subtracting for depreciation. “But that was so misleading,” he says.

He sued the insurer, alleging the company had acted in bad faith by seriously underestimating the amount of insurance he needed. The matter was settled out of court late last year.

Homeowners with unique, historical or custom-built houses might not be able to find adequate coverage from mass-market insurers, which typically pay to rebuild using only standard materials and construction techniques. These owners might have to turn to a specialty insurer, such as Chubb Corp., American International Group Inc. and Allianz AG’s Fireman’s Fund Insurance Co.