Mr Coles is expected to leave later this year and a search has begun for his successor which the BSA expects will take all year.
He first joined the BSA as an economist in 1979 and has worked as director-general since 1993.
Mr Coles said: "I have enjoyed, and continue to enjoy, being the BSA's director-general, a position I hold with considerable pride. However, now is the right time for me to seek to develop my career in other ways. I have held this post for 20 years, and it is time for a change, both from my point of view and that of the Association.
{desktop}{/desktop}{mobile}{/mobile}
"A new regulatory regime will be in place shortly and it is right that a new leader takes the BSA into this new age. Also, the mutual sector is recovering strongly after a difficult period in the financial and mortgage markets, giving my successor a very strong base from which to work."
David Webster, chairman of the BSA, said: "Adrian has been an outstanding ambassador for the mutual sector since joining the BSA as an economist in 1979, and he has played a pivotal role in guiding our industry through good times and tough times.
"Adrian has shaped the BSA into a robust, professional and respected organisation. He will be a tough act to follow and we wish him every success in his future career."

• Want to receive a free weekly summary of the best news stories from our website? Just go to home page and submit your name and email address. If you are already logged in you will need to log out to see the e-newsletter sign up. You can then log in again.