Gross lending by mutuals increased by 36% in March to £1.597m compared to £1.174m in February.
Figures from the Bank of England, which now combine building societies and mutual banks, showed that approvals were also stronger in March at £1.677m compared to £1.389m in February, a 21% increase.
Net lending by mutuals in March was £764m compared to £863m in February.
Adrian Coles, director-general of the BSA, said: "Lending activity appears to have gained some momentum following a sluggish start to the year and the relatively strong approval figures bode well for coming months.
"However, the mortgage market will remain fragile as there is uncertainty in relation to employment, interest rates, house price inflation, mortgage availability and, conceivably even after the election, the political outlook."
Went into administration April 2018
Threat of legal action looms over Woodford IM
View from the front row
Retirement Planner Forum 2019