Figures from the Building Societies Association (BSA) found this was higher than the £31.1bn completed in the six months to March.
Net mortgage lending rose by £11.7bn over the same period, making this part of the sector responsible for 72% of the market’s growth.
Further, the value of net mortgage lending surpassed growth seen across other lenders, which totalled £4.6bn.
In the year to September, mortgage balances at building societies increased by £16.5bn to £396.5bn, while balances at banks dropped by £1.3bn. The BSA said this meant building societies accounted for all growth in the mortgage market during the first nine months of the year.
More than 63,000 first-time buyers turned to a building society to get onto the property ladder in the six months to September, accounting for 44% of all residential owner-occupied lending.
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There were also 205,209 mortgage approvals across building societies from April to September, making up 32% of the market.
The mortgage arrears rate at mutuals was 0.32% at the end of the second quarter of this year, lower than the rate of 0.74% across the wider mortgage market.
Earlier this month, Chancellor Rachel Reeves announced the establishment of a Mutuals and Co-operative Council to drive growth across the building society sector.
