Higher interest rates and difficulty finding a competitive deal were the most common challenges at renewal.
Looking forward, 40% of borrowers plan to remortgage or take a product transfer in the next 12 months, covering around 2.5 loans each on average.
Among portfolio landlords with four or more BTL mortgages, around half expect to refinance in the year ahead, across an average of 3.7 loans.
Fixed rates remain the preference, with 2-year and 5-year terms equally popular.
28% are undecided on their next product type.
Two-thirds of landlords arranged their most recent BTL loan through a mortgage intermediary.
This rose to three-quarters for portfolio landlords.
When choosing a mortgage, landlords put the most importance on a competitive interest rate, followed by low upfront fees and charges.
Bethan Cooke (pictured), director at Pegasus Insight, said: “Buy-to-let is currently first and foremost a refinancing market, with landlords remortgaging and arranging product transfers at record levels.
“The point at which a fixed rate matures has become a pivotal moment in the lending relationship.
“Most landlords stay with their existing lender when their deal ends, but a significant minority look elsewhere, and because they begin researching their options months before expiry, there is a genuine window for lenders to engage early with competitive rates and low fees, the two things landlords tell us they care about the most.”
Cooke added: “For intermediaries, the picture is an encouraging one.
“Portfolio landlords in particular are managing multiple loans on different timelines and clearly value advice, and with deals maturing month after month, brokers who stay close to those clients as their fixed rates approach expiry are well placed to help them find the right deal.”