Average fixed mortgage rates in the U.S. fell for the second week in a row, according to mortgage-finance company Freddie Mac (FMCC), potentially increasing the affordability of buying a home as the key season for the housing market gets under way.

A series of data have been suggesting the key season for the housing market has been getting off to a slow start this year, including a February drop in pending home sales, a measure of early spring activity. Rising home prices and borrowing costs have been among the pressures on the market.

On Thursday, Freddie Mac Chief Economist Frank Nothaft noted in a statement that housing starts for March rose less than expected, while housing permits dropped 2.4% last month, and that the February tally was revised lower.

For the week ended Thursday, the 30-year fixed-rate mortgage averaged 4.27%, compared with 4.34% a week earlier and 3.41% a year earlier. Rates on 15-year fixed-rate mortgages averaged 3.33%, compared with 3.38% the previous week and 2.64% a year earlier.

Five-year Treasury-indexed hybrid adjustable-rate mortgages, or ARMs, on average, were 3.03%, compared with 3.09% the previous week and 2.6% a year earlier. One-year Treasury-indexed ARM rates on average were 2.44%, compared with 2.41% the previous week and 2.63% a year earlier.

Write to Tess Stynes at tess.stynes@wsj.com

Subscribe to WSJ: http://online.wsj.com?mod=djnwires