This was the highest proportion that Paragon has recorded since it began asking the question in Q1 2007.
By comparison, residential owner-occupied mortgage business fell to its lowest level, accounting for 66.6% of all applications, down from 72.2% in the second quarter.
On average, brokers saw their buy-to-let business increase 3.1% during the third quarter.
Overall, Paragon’s survey found that 43% of brokers recorded a rise in their buy-to-let business levels in Q3.
Of these brokers, 12% said business levels rose by more than 10%, with 13% stating buy-to-let business was up between 6% and 10%.
However, almost half of intermediaries said buy-to-let business levels remained unchanged during the period, while 7% saw a drop.
Elsewhere, intermediaries reported an improvement in the availability of buy-to-let finance, with 58% of respondents saying they believed buy-to-let mortgages were more readily available and 31% saying that availability had stayed the same.
John Heron, managing director of Paragon Mortgages, said: “It is positive to see that buy to let accounted for a growing percentage of intermediaries’ overall business levels during the third quarter.
“Given the pressures on the private rented sector, it is important that landlords continue to be active purchasers and develop their portfolios.”
Heron added: “There are plenty of opportunities for intermediaries to develop further business in this market, but more needs to be done to deliver products and advice to larger scale investors.
“Cultivating a relationship with professional landlords can generate repeat business for intermediaries as well as ancillary sales.”
