If you earn an average salary, you likely want to live somewhere you can comfortably afford your necessities and still have money leftover to put away for emergencies, retirement and leisure. 

Unfortunately, that's a somewhat rare situation for families in the U.S. Just 18% of the 100 largest counties have a median income large enough to support a family of two adults and one child, according to a recent SmartAsset analysis. 

Fairfax County, Virginia, is home to much of the Washington D.C. suburbs and with that, many highly-paid government, financial services and tech workers. The area is quite costly, however, leaving a family of three just under $30,000 a year for nonessentials, the most of any of the largest 100 U.S. counties, according to SmartAsset.

The financial services platform used income data from the University of Wisconsin's County Health Rankings program and cost of living estimates from Massachusetts Institute of Technology's Living Wage Calculator to determine how much money average working Americans can expect to have left after their needs are met.

Here are the counties where families of two adults and one child earning the median household income have the most left over after basic expenses, according to SmartAsset:

1. Fairfax County, Virginia

• Disposable income: $29,942
• Median household income: $144,632

2. Collin County, Texas

• Disposable income: $20,801
• Median household income: $113,943

3. Santa Clara County, California

• Disposable income: $20,377
• Median household income: $150,502

4. Fort Bend County, Texas

• Disposable income: $15,769
• Median household income: $105,583

5. Nassau County, New York

• Disposable income: $14,306
• Median household income: $135,528

6. Montgomery County, Maryland

• Disposable income: $12,606
• Median household income: $118,020

7. Denton County, Texas

• Disposable income: $12,481
• Median household income: $102,711

8. Montgomery County, Texas

• Disposable income: $9,254
• Median household income: $95,241

9. Bergen County, New Jersey

• Disposable income: $9,136
• Median household income: $113,885

10. Utah County, Utah

• Disposable income: $8,640
• Median household income: $95,085

In all but two of these counties, median household income is over $100,000, yet families don't have much left over after their essential costs are paid for.

Still, it may be worth it to stay in a high-cost area if it helps your career, Jen Swindler, a certified financial planner based in Salt Lake City, Utah, tells CNBC Make It.

"Typically, my high-cost area and high salary clients have a lot more savings and disposable income, even though it's expensive to live in the city in question," she says. "The job opportunities usually make up more than the difference."

Local living costs and income aren't always perfectly proportional, though. Some families may find an area where the local pay is marginally higher, but the cost of living is significantly higher, eating into any extra earnings.

On the flip side, a place could seem attractive with a low cost of living, but if you can't bring a large salary with you as a remote worker and local salaries are correspondingly low, the savings may not be there.

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