While 401(k)s are known for their limited menu of options, more and more plans have been adding an escape hatch—or more precisely, a window. Known as a “brokerage window,” this plan feature gives you access to a brokerage account, which allows you to invest in wide variety of funds that aren’t part of of your plan’s regular menu. Some 401(k)s also allow you to trade stocks and exchange-traded funds, including those that target exotic assets such as real estate. No question, brokerage windows can be a useful tool for some investors. But these windows carry extra costs, and given the increased investing options, you also face a higher risk that you’ll end up with a bad investment. All of which raises concerns that many employees may not fully understand what they’re getting into with these accounts. Earlier this week the U.S. Labor Department, which has been fighting a long-running battle to make retirement plans cheaper and safer for investors, asked 401(k) plan providers for information about brokerage windows. You may wonder if a brokerage window is something you should use in your 401(k). To help you decide, here are answers to three key questions: How common are brokerage windows? Not long ago these features were rare. As recently as 2003, just 14% of large plans included offered a brokerage window, according to benefits company Aon Hewitt. But they’ve grown steadily more popular over the past decade, with about 40% of plans offering this option as of 2013. Interestingly, the growth has taken place even as more 401(k)s have opted to take investment decisions out of workers’ hands by automatically enrolling them in all-in-one investments like target-date funds. Those two trends aren’t necessarily at odds. Experts say companies often add brokerage windows in response to a small but vocal minority of investors, who, rightly or wrongly, believe they can boost returns by actively picking investments. But overall just 5.6% of 401(k) investors opt for a window when it is offered. The group that is most likely use a brokerage window: males earning more than $100,000, about 9% of whom take advantage of the feature, according to Hewitt. (Not surprisingly, this group also tends to have the corporate clout to persuade HR to provide this option.) By contrast, only about 4% of high-earning women use a window. When can brokerage windows make sense for the rest of us? That depends in part on whether the other offerings in your 401(k) meet your needs. If you want an all-index portfolio, for example, a brokerage window may come in handy. Granted, more plans have added low-cost index funds, especially if you work for a large or mid-sized company. Today about 95% of large employers offer a large-company stock index fund, such as one that tracks the S&P 500, according to Hewitt. Workers at small companies are less likely to enjoy the same access, however. These index funds are on the menu only about 65% of the time in plans with fewer than 50 participants, according to the Plan Sponsor Council of America, a trade group. Moreover, even in large plans investors seeking to diversify beyond the broad stock and bond market can find themselves out of luck. Only about 25% of plans offer a fund that invests in REITS. And only about two in five offer a specialty bond fund, such as one that holds TIPS. But even if a window allows you to diversify, you need to consider the additional costs. About 60% of plans that offer a brokerage window charge an annual maintenance fee for using it, according to Hewitt. The average amount of the fee was $94. And investors who use the window typically also pay trading commissions, just like they do at a regular brokerage. Where does that leave me? Before you decide to opt for a brokerage window, check to see if the fees outweigh the potential benefits. Here are some back-of-the-envelope calculations to get you started: If you have, say, $200,000 socked away for retirement, paying an extra $100 a year to access a brokerage window works out to a modest additional fee of 0.05%. While the brokerage commission would increase that somewhat, you can minimize the damage by trading just once a quarter or once a year. If your plan includes only actively managed mutual funds with annual investment fees in the neighborhood of 1%, the brokerage window could allow you to access ETFs charging as little as 0.1%. That means you could end up paying something like 0.15% instead of 1%. If your plan has low-cost broad market index funds, however, a brokerage window offers less value. Say you want to add more more specialized investment options, such as a REIT or emerging market fund. Even if you have $200,000 in your 401(k), you’ll probably only invest a small amount in these more exotic investments—perhaps $5,000 or $10,000. So a $100 brokerage fee would increase your overall costs on that slice of your portfolio to 1% to 2%. Plus, you’ll pay brokerage commissions and fund investment fees. In that case, better to leave the escape hatch shut.