When preparing for retirement, saving enough to cover just basic living expenses can be tough. Retirement will likely last at least a couple of decades, and spending only $20,000 or $30,000 per year can amount to several hundred thousand dollars in that timeframe. Then if you want to travel or enjoy other expensive luxuries in retirement, that figure can easily skyrocket.

It also doesn't help that the average worker is behind on their retirement savings, with nearly half of baby boomers reporting they don't have anything saved at all, according to a survey from the Insured Retirement Institute. When you're struggling to save in the first place, saving even the bare minimum to get by during retirement can be a challenging task.

But while most people think of the "bare minimum" as expenses like housing, food, and transportation, there are several major costs most retirees will face that not everyone prepares for. And by not accounting for these expenses in your retirement budget, you could be in for a nasty surprise down the road.

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1. All the costs involved with Medicare coverage

A whopping 72% of Americans admit they don't fully understand how Medicare works, according to a survey from the Nationwide Retirement Institute, and 53% mistakenly believe coverage is free. If you're not aware of what you could be paying for healthcare, these costs can take a big bite out of your budget.

With Original Medicare, Part A (which covers most hospital visits) is typically free to those who have worked and paid Medicare taxes for at least 10 years. But Part B, which covers doctor visits, requires a monthly premium of around $135 per month (though high earners could have to pay more than that). Original Medicare also doesn't cover most routine care (such as dental and vision care), so these costs will likely need to be paid out-of-pocket. Prescription drug coverage is another area not covered by Original Medicare, although you can enroll in Part D coverage at an additional cost.

Many retirees instead choose a Medicare Advantage plan, which provides coverage similar to what you likely received through your employer. However, these plans are often more expensive than Original Medicare because they offer more expansive coverage.

Regardless of which type of plan you choose, healthcare is far from free in retirement. You're still responsible for all premiums, deductibles, and coinsurance. And the average retiree spends around $4,300 per year on out-of-pocket healthcare expenses, according to a study from the Center for Retirement Research at Boston College. 

2. Long-term care costs

Approximately 70% of older adults will require long-term care at some point during retirement, the U.S. Department of Health and Human Services found. And of those who need long-term care, around one in five people will need it for longer than five years.

This type of care can also cost a pretty penny. For nursing home care, the monthly cost of a semi-private room will run around $6,800, and a private room costs an average of $7,700 per month, according to the Department of Health and Human Services. Furthermore, long-term care typically isn't covered by Medicare. While Medicare will sometimes cover short stays in a skilled nursing facility if the care is considered medically necessary, it doesn't cover custodial care -- which involves help with daily activities like dressing, bathing, and eating.

Because most long-term care can be incredibly expensive and isn't covered by Medicare, it can quickly drain your retirement fund. One potential solution is to buy long-term care insurance, but it's important to enroll before you retire. Premiums are notoriously high, but they become even higher if you wait until your 60s or later to sign up. Wait too long, and you might be denied coverage altogether.

The average 55-year-old couple can expect to pay around $2,500 per year for long-term care insurance, according to the American Association for Long-Term Care Insurance, while the average 60-year-old couple will face premiums of around $3,400 per year. These rates are expensive, without a doubt. But considering a 5-year stay in a nursing home can cost more than $400,000 without insurance, it might be worth it to pay for insurance now rather than face hefty out-of-pocket expenses later.

3. Taxes on retirement account withdrawals

When you're figuring out how much you should be saving for retirement, you're probably thinking about after-tax dollars. But if you're stashing your money in a 401(k) or traditional IRA, you'll need to pay income taxes on the money you withdraw during retirement -- so you may not have quite as much cash to spend as you think.

Not accounting for taxes on withdrawals can throw off your entire retirement plan. For example, if you expect to need $40,000 per year in retirement and you save enough so that you can withdraw exactly that much from your savings, you might be in for a surprise if you're left with, say, $32,000 after taxes. If that's not enough to cover all your expenses in retirement, you may be forced to withdraw more each year than you'd planned -- which puts you at risk of running out of money sooner than you expected.

It's a good idea to account for taxes as you're estimating your retirement costs and creating a savings plan. If you save more than you think you need, assuming part of your savings will go toward taxes each year, you won't be in for a surprise when the IRS wants a chunk of your retirement withdrawals.

You may also choose to invest some of your savings in a Roth IRA, which taxes your contributions upfront so you don't need to pay income tax on withdrawals. There is a potential downside to a Roth IRA, though, if you're a high earner now and expect to be in a lower tax bracket come retirement age. In that case, you could end up paying more in taxes upfront than you would if you'd paid income taxes on withdrawals.

It's difficult to save for retirement, because nobody can predict exactly how much they'll need to cover all their expenses. But there are some costs that can come back to bite you if you're not preparing for them ahead of time. By doing your homework and thinking about how you'll pay for these major expenses, you'll put yourself in a much better position to retire comfortably and ensure your savings last as long as possible.