The 12 least affordable housing markets in the U.S.

There seems to be some cooling off in the housing market. But that doesn't mean homes are becoming more affordable everywhere.

According to RealtyHop's housing affordability index, many cities in the U.S. are still difficult to afford. Out of the 100 cities in the index, the average American would have to dedicate at least 40% of their income to own a home in 42 of them. 

The index takes into account median household income, median for-sale prices using RealtyHop listings, local property taxes using American Community Survey (ACS) Census data and mortgage expenses. Projected mortgage expenses assume a 30-year mortgage, 5.5% interest rate and 20% down payment.

Here are the 12 least affordable housing markets in the U.S., as well as the median household income, the median home price and the percentage of income required to afford homeownership in each place.  

1. Miami

• Median household income: $44,581
• Median home price: $610,000
• Share of income: 87.39%

2. Los Angeles

• Median household income: $69,695
• Median home price: $975,000
• Share of income: 85.34%

3. New York 

• Median household income: $68,129
• Median home price: $925,000
• Share of income: 82.47%

4. Newark, New Jersey 

• Median household income: $38,854
• Median home price: $385,000
• Share of income: 77.52%

5. Hialeah, Florida 

• Median household income: $40,036
• Median home price: $465,000
• Share of income: 72.55%

6. Long Beach, California 

• Median household income: $70,677
• Median home price: $799,000
• Share of income: 69.77%

7. San Francisco

• Median household income: $126,117
• Median home price: $1,388,000
• Share of income: 66.56%

8. San Diego

• Median household income: $89,357
• Median home price: $950,000
• Share of income: 65.65%

9. Anaheim, California 

• Median household income: $80,486
• Median home price: $834,250
• Share of income: 63.98%

10. Santa Ana, California 

• Median household income: $74,185
• Median home price: $750,000
• Share of income: 62.14%

11. Oakland, California 

• Median household income: $82,649
• Median home price: $798,000
• Share of income: 60.85%

12. Boston

• Median household income: $79,797
• Median home price: $775,000
• Share of income: 59.38%

In order to be homeowners, an average family living in Miami, Los Angeles or New York would have to dedicate over 80% of their annual income to housing. The remaining 20% would need to be enough to cover every other expense, which is likely unsustainable.

Kevin O'Leary, judge on CNBC's "Money Court," advises potential homeowners to follow the ⅓ rule when buying. This means that only ⅓ of your income after taxes should go toward your home. Anyone earning the median income in any of the places above would be well outside of that rule.

Instead, "you may have to live in a smaller apartment if you're renting, or buy a smaller home to start," O'Leary previously told CNBC Make It.

O'Leary's rule is similar to advice first issued by the U.S. government in 1981 stating that you should aim to spend no more than 30% of your income on housing costs, including mortgage interest, property taxes and maintenance.

Sign up now: Get smarter about your money and career with our weekly newsletter

Don't miss: Is the job market really getting worse? 3 recent job-seekers weigh in