The figure was the highest on record for a single quarter since the start of the broker’s Complex Buy to Let Index in Q1 2011.
The research also found that refinancing also accounted for over two-thirds of transactions on Houses in Multiple Occupation and Multi-unit Freehold Blocks.
Mortgages for Business attributed the rise to increases in average gross yields over the past year.
Yields on vanilla property have improved marginally from 6.3% to 6.4% over the past 12 months. HMO yields have also fallen from 10.7% to 10.5% over the past 12 months but it still remains the highest yielding property type.
David Whittaker, managing director of Mortgages for Business, said: “Gross yields are tantalisingly strong at the moment and that has sparked a real splurge of refinancing as landlords try to unlock enough capital to expand their portfolios and make hay while yields are high.
“With so much refinancing going on at the moment, we might well see a purple patch of purchasing activity later on in 2013.”
Average property values for buy to let transactions increased over the course of the year on residential stock and MUFB.
The number of mortgages on the market fell for the second quarter in succession down from 444 to 434 as lenders look to cull unpopular mortgages from their range.
David Whittaker said: “The range of mortgages shrunk over the last quarter but that’s more because lenders are being selective and weeding out less popular products, rather than a sign that it is becoming harder for property investors to get a mortgage.”