The mutual will now assess loans at rental coverage of 135% with an interest rate of 5.5% – down from 5.99%.
Borrowers can also consider other income to support the application.
The society has also added a new buy-to-let affordability calculator including the rate change and taking into account the potential top-slicing.
It will also help with top-slicing calculations for any regulated, consumer, ex-pat and limited company applications.
Newbury Building Society lending manager Roger Knight said the calculator quickly informs the broker if their client is suitable or not.
“This means that we will consider income from a day job rather than rent to boost the amount an applicant can borrow,” he said.
“We do this by considering the borrower’s overall finances, including income and expenditure. If we’re comfortable with sufficient surplus income, we will use this to top up the rental calculation to increase the amount that can be borrowed,” he added.