There's a reason so many people choose to save for retirement in a 401(k) or individual retirement account (IRA). These accounts offer some pretty solid tax breaks.

Traditional IRAs and 401(k) plans are funded with pretax dollars, allowing you to shield some income from the IRS. These accounts also allow your money to grow on a tax-deferred basis, so you don't have to pay capital gains taxes every year. Rather, taxes are due only once you take withdrawals.

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But while it's a good idea to use a 401(k) or IRA to build a retirement nest egg, you shouldn't keep all your long-term savings in one of these accounts. Doing so is actually riskier than you might expect.

The problem with sticking to just a 401(k) or IRA

It's easy to see why you may be inclined to only use a 401(k) or IRA to save for retirement. Why not get a tax break in the course of building savings?

But a big problem with these accounts is that they come with restrictions. And one restriction in particular could put you in a tough spot if you end up having to retire early.

With a 401(k) or IRA, you must leave your money untouched until age 59 1/2. If you take money out of a 401(k) or IRA before that age, you'll generally face a 10% early-withdrawal penalty.

Now, you might say to yourself, "Fine, then I'll just make sure to work until 59 1/2 and leave that money alone." But what if things don't work out that way?

You never know when you might end up having to retire early. Your company might have layoffs. Your industry might collapse. You may encounter health issues that make it harder to do your job. Or someone you love might get hurt or sick, forcing you to become a caregiver.

The problem is that the IRS doesn't care if you end up retiring early not because you want to, but because you have to. And if you have all your savings in a 401(k) or IRA and you need to withdraw funds before turning 59 1/2, you risk facing a huge penalty due to circumstances that may be outside your control.

A taxable brokerage account gives you more flexibility

Even though it makes sense to use a 401(k) or IRA to build retirement savings, it's a good idea to keep a portion of your nest egg in a taxable brokerage account. Your money won't go in tax-free, and you'll have to pay taxes on capital gains and dividends in your account yearly.

But what you get in return is flexibility. If you need to take money out at age 50 to cope with a major home repair, you can. If you need to retire at 57 because there's no work to be found in your field, you won't be penalized for tapping your own savings.

What's also helpful is that a taxable brokerage account won't force you to take withdrawals in retirement. A 401(k) or IRA will.

Once you turn 73 or 75, depending on your year of birth, traditional 401(k)s and IRAs force you to take required minimum distributions (RMDs). Those could drive up your taxes and have other consequences.

A move that makes sense for your peace of mind

There's nothing wrong with keeping the bulk of your retirement nest egg in an IRA or 401(k). But you may want to keep a decent portion of your savings in a taxable brokerage account so you have more options.

Remember, a 401(k) or IRA will penalize you for taking money out too early and also penalize you if you don't take money out at a certain point to fulfill your RMDs. That's a pretty raw deal, despite the tax breaks these accounts offer. So it's important to look to a taxable brokerage account to offset these strict rules.