Nationwide has reported a record amount of mortgage lending for a six-month period and a 34% rise in profits.
For the half year to 30 September, lending was up 14% to £14.9bn, and profits rose to £802m
However, it said that it expected "downward pressure" on its profit margins in its second half and next year due to "robust" competition.
It added that incoming chief executive Joe Garner will take up his position in the spring.
"This has been our best ever half year of mortgage lending along with a strong inflow of savings and the opening of over a quarter of a million new current accounts," said chief executive Graham Beale.
"Mutuals, like Nationwide, are different from the banks... Nationwide is evidence that you can be successful by doing the right thing," he added.
The building society said in its outlook that healthy employment growth and "robust demand from investors" - including buy to let - had driven up prices, especially in London.
But it warned that "such outperformance is unlikely to be sustained over the long term, given that key measures of affordability are already stretched".