Slightly looser underwriting outside the government sector is primarily responsible for the latest increase in credit availability.

Credit availability increased to 181.4 in September from 108.2 in August and 167 a year ago, according to the Mortgage Credit Availability Index, which is based on the Mortgage Bankers Association's analysis of data from mortgage technology provider Ellie Mae.

"Mortgage credit availability increased in September due to continuing updates to conforming loan programs as well as agency jumbo programs that have been phased in over the last few months," Lynn Fisher, the MBA's vice president of research and economics, said in a press release.

The components of the index that measure loan programs outside the government sector generated the strongest consecutive-month gains of around 1.4% or 1.5% in September.

Underwriting loosening in the jumbo sector was on the low end of that range in the past month but over the course of the year it's been the sector where credit availability has increased the most.

"For the year to date, the supply of credit has increased only modestly in the non-jumbo space while it has expanded significantly among jumbo programs," said Fisher.

Government programs only saw a 0.2% increase in credit availability between August and September.

Overall credit availability for this year peaked at 183.4 in March following a surge in jumbo adjustable-rate mortgage programs. A subsequent reduction in conforming offerings in April and decline in government loan product offerings in May reduced overall credit availability until more loosening in the jumbo and ARM sectors during July and August, respectively, increased overall credit availability again.

The MBA last updated the methodology used to create the index in August of last year.

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