The mutual has also doubled its total individual borrowing limit from £500,000 to £1m and will lend when up to 15 mortgages remain outstanding with other lenders.
In January the society increased its loan to value (LTV) for landlords to 75%.
National development manager Steve Walton said the latest step was in response to house price inflation over recent years.
“We’re taking these measures to make our individual underwriting proposition more exciting and available to larger portfolio landlords on higher value housing stock,” he said.
“Landlords need more choice, especially given the government’s reduction in tax relief and the regulatory change to rental income calculations,” he added.