Rising customer numbers during the year took the society's membership to record levels and savings balances were greater than at any time in its history as profits leaped 34% to £108.5m.
Mortgage completions during the year also reached their highest ever level at £3.1bn, taking gross lending in the past five years to almost £11bn.
Chief executive officer Peter Hill said: “I'm proud to report another excellent set of results after a successful 12 months, building on the sustained growth in recent years.
"We achieved this by providing good value products, despite the historically low interest rate environment.
“Increasing our lending led to record levels of profit, which enabled us to increase our capital and reserves and invest further in the long-term financial strength of the society.
"As we celebrated our 140th anniversary in 2015, we helped more people than ever before to save for the future and to have the homes they want.
"And in the spirit of our founders, we continue to move with the times and innovate, to keep improving the service we offer to members.”