Building society gross advances hit £4.2bn in July -the highest figure since August 1988.
The figures represent a substantial rise from June's gross advances which totalled £3.94bn and £3.6bn in July last year.
Net advances were £1.58bn in July, up from £1.22bn a year ago, while approvals rose to £4.29bn from £3.79bn.
Net inflows remain low, totalling £155m in July, up from last month's £65m but down from £879m a year ago.
BSA director general Adrian Coles says: “Building societies' success in the mortgage market continues, with gross advances at the highest figure since August 1988, when the building society sector was much larger.
“Savings inflows into building societies have fallen away in recent months, possibly as a result of further falls in the general level of interest rates, competition from National Savings and the modest recovery in the stockmarket.”