In retirement, which expenses will take the biggest bite of your income? Those about to retire think they know the answer. But most of them are wrong.

A recent Harris Poll found that among people 55 and older who were still working, most believed health care costs would be their biggest expenditure in retirement. The same poll also asked people 65 and older who were retired what their biggest expense was. Their No. 1 answer was paying their monthly housing bills.

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In fact, for most people, housing-related costs will outstrip health care — or any other expenses — both before and during retirement, according to "Expenditure Patterns of Older Americans," a report by the Employee Benefit Research Institute, a nonpartisan public policy organization.

"Housing is the top [cost], but it more or less remains flat," says Sudipto Banerjee, author of the report. "Health care comes second, but it is the only category that increases with age." Retired households, on average, are getting by on 57 percent less income than nonretired households. Their expenditures, meanwhile, are about 80 percent of a nonretired household.

The report found that people 50 and older spend 40 to 45 percent of their income on home-related expenses. The percentage falls slightly as people age, mainly as many pay off their mortgages. Banerjee said the percentages are fairly consistent throughout the country. On Long Island, for example, higher household income is offset by higher property taxes. In addition to mortgage and taxes, home-related expenses include insurance, rent, utilities, repairs and furnishings.

As for health care costs, the report found it accounts for about 10 percent of the expenditures for people ages 50-64, but by the time a person hits 85, the costs surge to about 20 percent of the monthly budget. "The rate of health costs is increasing faster than inflation, and as you age you are using more and more health care," Banerjee says.

Not surprisingly, those with the lowest income are having a hard time paying their expenses, especially when they hit their mid-70s. And while the study found that higher-wealth households currently "are managing their income and expenses well in retirement," there is reason for concern, especially as the population lives longer. What remains unclear, the study concluded, is whether they will have enough resources to support themselves through "very advanced ages."