It's no secret that retirement is expensive. The average U.S. adult age 65 and older spends around $46,000 per year, according to the Bureau of Labor Statistics, and after a couple of decades in retirement, you could easily spend hundreds of thousands of dollars.

However, there are some costs you may not be considering that could make retirement even more expensive. And if you're not accounting for these hidden costs, your savings may not last as long as you think.

Senior couple sitting at a table looking worried

Image source: Getty Images.

1. High 401(k) fees

Even if you don't realize it, you're paying fees on your 401(k) investments. Every retirement account charges fees, but some plans have higher fees than others.

The average 401(k) plan charges an annual fee of 1% of assets under management, according to a report from the Center for American Progress. So, for instance, if you have $100,000 in your 401(k), you'd be paying $1,000 per year in fees.

That may not sound like much, but when your money has been invested for decades, those fees add up. The average worker paying 1% in annual 401(k) fees will pay more than $138,000 in fees alone over a lifetime, according to the Center for American Progress. But if your annual fees increased to just 1.3% per year, you'd end up spending more than $166,000 in fees over a lifetime.

To see how much you're paying in fees, check your plan statements or talk to your plan administrator. If you find that you're paying higher-than-average fees, you might save thousands of dollars by switching to a different type of retirement account.

2. Long-term care costs

Nearly three-quarters of seniors will need long-term care eventually, according to the U.S. Department of Health and Human Services. If you're lucky, you may be able to rely on loved ones to take care of you as you age. But not everyone has that privilege, and long-term care comes with a hefty price tag.

The average semi-private room in a nursing home will run you more than $82,000 per year, according to the Department of Health and Human Services, and Medicare typically won't cover long-term care. So if you're not preparing for this expense, you could be in for a costly surprise.

One way to reduce your out-of-pocket expenses is to enroll in long-term care insurance. Although premiums can be high, the earlier you sign up, the less you'll pay each year. And high premiums may still be preferable to paying tens or even hundreds of thousands of dollars in out-of-pocket long-term care costs.

3. Retirement taxes

Taxes don't go away once you leave the workforce, and you may have to pay taxes on your retirement account withdrawals as well as your Social Security benefits.

Withdrawals from a 401(k) or traditional IRA will be subject to income taxes. In addition, you'll need to start taking required minimum distributions (RMDs) from these types of accounts once you turn 72 years old -- even if you're still working at that age. If you fail to take your RMD, you'll be slammed with a tax penalty of 50% of the amount you were required to withdraw.

Your Social Security benefits may not escape taxation either, and how much you owe will depend on what's called your "combined income." Your combined income is half your annual benefit amount plus your adjusted gross income, and if it's more than $25,000 per year (or $32,000 per year for married couples filing jointly), you'll owe taxes on at least a portion of your benefits.

Don't let these hidden costs derail your retirement

Planning for retirement is tough, especially when you consider how many expenses you'll need to prepare for. But the more you're able to plan for both the expected and unexpected costs, the better off you'll be in retirement.