Building society mortgage approvals totalled £57.8 billion in 2015, a rise of 10% by value over the year.
The mutual sector now accounts for 26% of all approvals, according to latest figures from the Building Societies Association.
The number of approvals rose 6% to 395,300, of which around a third were to first-time buyers.
Market share by number of loans approved was 28%.
Net lending for 2015 fell to £15.2 billion from £17.3 billion in 2014 but building societies’ market share remains strong, accounting for 45% of all net lending in the year.
At the end of 2015, building societies held mortgages to the value of £265.2 billion, 21% of the UK market.
Paul Broadhead, head of mortgage policy at the BSA, said: “Competition in the mortgage market picked up in 2015 with the large banks demonstrating an increased appetite to lend.
“Despite this, building societies have continued to benefit from their ability to provide mortgages to consumers with a wide range of requirements, including first-time buyers, self-builders, shared owners and those needing mortgages later in life."
Broadhead added: “Despite European and global uncertainty, consumer sentiment in the housing market remains strong and building societies will continue to have a compelling mortgage lending proposition during 2016.
"However, we do remain concerned about consumer affordability across all housing tenures due to the continued shortage of housing supply.”