The lenders made an incremental increase in the market compared to the same period last year when their share was 26 per cent, according to figures from the Building Societies Association (BSA).
Around 111,000 mortgages were approved over the three months to the end of September, a five per cent increase on the 106,200 deals in the same quarter of 2019.
However gross lending was £14.6bn, down nine per cent on the £15.9bn a year prior, while net lending shrank by a significant 43 per cent to £1.2bn from £2.1bn.
Building societies hold outstanding mortgage balances of £338.0bn, a 23 per cent market share, and up one per cent on the £333.9bn at the end of Q3 2019.
More than 20,000 first-time buyers received mortgages from the mutuals.
At the same time, savings balances increased by £1.9bn and were up 40 per cent on the £1.3bn increase in Q3 2019.
Paul Broadhead, head of mortgage policy at the BSA (pictured), said: “It has been a turbulent year for the mortgage market, with transactions collapsing due to the lockdown in March, but approvals for house purchase recovering to 10 year highs in the third quarter, as pent up demand was released and buyers rushed to take advantage of the stamp duty holiday.
“Building societies have been able to support homebuyers during this period, approving 28 per cent of all new mortgage loans in the third quarter of the year.
“Although the housing and mortgage markets are buoyant at the moment, and the wider economy has recovered somewhat, we are far from out of the woods.
“Around nine per cent of the workforce (3.3 million jobs) are currently being supported by the furlough scheme, and only once this and other government support has ended will the long term impact of coronavirus be fully understood.
“We are also concerned about the cliff edge effect of the abrupt end of the stamp duty holiday on 31 March and have called on the government to taper its removal to lessen market impact.”