It costs more than two thirds of the average annual income to cobble together a 20 percent down payment on a home, which is why many potential homebuyers say saving for a down payment is one of their top concerns.

In the U.S., to buy a median-priced home for $192,500, a buyer must scrape together $38,500 in cash, plus any closing costs and moving expenses, in order to ensure they qualify and get the best terms on their mortgage, Zillow says in its study.

Among large housing markets, buyers in the San Jose, San Francisco and Los Angeles metros must come up with the largest percentage of their income to buy a home – 182 percent of the average annual income to put 20 percent down on the median home.

In San Jose, where the median income is $105,455, a down payment on the median $961,600 home is $192,320.

Buyers in Pittsburgh, Indianapolis, and Kansas City set aside the smallest chunk of their income for the ideal down payment: 48 percent of the median annual wage.

Should mortgage interest rates rise in 2017 as expected, a solid down payment will become increasingly important. While it is possible to put down as little as 3.5 percent on a home, the trade-off is a higher interest rate and costly private mortgage insurance. A better interest rate can translate to thousands of dollars over time; on a $200,000 loan, lowering the interest rate by half a percentage point will save $20,000 over the lifetime of the loan.

"Saving enough cash for a down payment is a major barrier to homeownership, especially in expensive markets, where a 20 percent down payment can cost nearly $200,000," said Zillow Chief Marketing Officer Jeremy Wacksman. "While it's possible to buy a house with a smaller down payment, 20 percent ensures the best rates. As important as it is to find a monthly payment you can afford, some buyers' budgets will come down to the amount of cash they can bring to the table."

Nearly half of all home buyers are buying a home for the first time, according to the Zillow Group Report on Consumer Housing Trends. Those buyers can't rely on the equity in their current home and must come up with the cash to get into the housing market – often while paying record-high rents. First time home buyers are also more likely to have received a financial gift to help with their down payment.

One in five home searchers said saving for a down payment is their top concern about the home-buying process – second only to finding an affordable home, which was their top concern.  Most – 56 percent – saved for their down payment the old-fashioned way, setting aside a little money at a time, according to the Zillow Group Report.

One third of buyers used more than one source of funds for their down payment, combining savings with gifts and loans from family and friends or cashing in their retirement.