The Mortgage Lender Benchmark by Smart Money People consisted of 597 brokers reporting on 44 lenders across banks, building societies, specialist lenders and lifetime providers.
It found complex buy-to-let lenders were rated 96 per cent for their flexibility.
Mainstream buy-to-let lenders followed, with a score of 87 per cent for their tolerance with mortgage applications.
Lenders across all categories were scored 79 per cent for their flexibility.
Brokers rated the underwriting from complex buy-to-let lenders higher than mainstream, with a score of 50 per cent compared to 32 per cent. The overall average across all lenders was 51 per cent.
However, complex lenders did not do too well when it came to how quickly mortgage applications were processed, likely due to the more intensive underwriting.
This was evident in how brokers rated complex buy-to-let lenders for speed, which was given a score of 29 per cent compared to the 55 per cent rating for mainstream buy-to-let lenders.
Again, potentially due to the nature of these cases brokers said they felt mainstream leaders were easier to place applications with. Mainstream buy-to-let lenders were rated 80 per cent for their ease, while complex lenders were scored 67 per cent.
Jacqueline Dewey, CEO of Smart Money People, said: “It’s clear that brokers realise and appreciate the complex nature of non-mainstream buy-to-let cases and how lenders approach them, especially when it comes to how flexible a lender is willing to be for these cases.
“However a rating of 29 per cent for speed compared to the overall lender average of 58 per cent shows that complex buy-to-let lenders still have room for improvement in their backend processes.”