Having an extra bedroom is a popular waste of space. It is often just a lightly used home office, or the guest bedroom that is occupied a grand total of maybe two weeks a year, or the bedroom for your kid who moved out eight years ago, but you don’t dare downsize because of the three weekends he visits each year. He enjoys having his room.

An analysis by Finder.com notes that we are a nation with more bedrooms (357 million) than people (323 million).

That spare bedroom comes at a mighty cost. Realtyhop, a tech-driven real estate firm catering to investors, notes that across the 100 largest U.S. metro areas, the median price for a four-bedroom home is $100,000 more than the median price for a three-bedroom home.

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That’s six figures of motivation to think about giving up the spare. The financial windfall can be an eye-opener. Even if you graciously subsidize the Airbnb rental your visitors will use, you are going to come out ahead.

For example, in the metro Boston area, the median price for a three-bedroom last year was around $380,000 while a four-bedroom cost $540,000. The monthly mortgage differential — assuming a 20% down payment and a 30-year fixed rate loan at 4% — is more than $600 a month.

And $7,200 a year is a lot of nights in a nearby Airbnb, far more than you’re likely to need. And we haven’t even started factoring in the lower property tax bill and lower heating and cooling costs if a home with fewer bedrooms means a smaller overall footprint.

Even in less expensive areas, a step down in bedrooms saves plenty. In Las Vegas, Realtyhop says the $280,000 median cost for a three-bedroom is $80,000 more than what a two-bedroom runs. In Chicago, the three-bedroom median of $240,000 is nearly $100,000 less than a four-bedroom.

Giving up a little (lightly used) space can finance a whole lot of interesting lifestyle choices. For instance, using the Boston example, that $600 a month can make it possible to stay in a beloved neighborhood or school district without busting the budget. Perhaps it’s the financial freedom to turn down a more demanding job that buys you more family time, because you’re not groaning under the weight of the bigger mortgage payment.

Or it’s the gateway to retiring earlier, not later. Buy the one-fewer-bedroom house, invest the $600 a month in savings and earn an annualized 5%, and your retirement accounts will be $500,000 bigger by the time the mortgage is paid off in 30 years. That’s how you retire by betting on the house, so to speak.

For baby boomers thinking of moving, focusing on two-bedrooms rather than three can buy you more retirement security. In Miami, opting for a median priced two-bedroom, rather than three-, saves you $100,000. In San Diego it’s $150,000 less. In Seattle, the two-bedroom saves you $88,000. Maybe you tuck that money into your savings account for years of peaceful sleeping. Or maybe it’s the money to travel more, or book some bucket-list trips with the kids and grandkids.

For younger families, keep in mind that a separate bedroom for each kid is not a rule of good parenting. It’s just a late 20th century convention, adopted as homes got bigger and now has become some sort of norm in many households. Just saying.

Carla Fried is a freelance personal finance journalist. Distributed by Tribune News Service.