Mr Coles, 58, joined the association as an economist in December 1979, and took over the director-general role in July 1993. He will continue to lead the body - which represents the interests of mutual lenders and deposit takers across the UK, including all 47 UK building societies - until a successor is found towards the end of the year.
Commenting on his departure, BSA chairman David Webster said: “Adrian has been an outstanding ambassador for the mutual sector since joining the association and has played a pivotal role in guiding our industry through good times and tough times.
“He is hugely respected, not only in the UK, but amongst financial services businesses overseas for his perceptive grasp of economic, political and regulatory issues and his ability to distil such complex topics into meaningful and impactful priorities.
“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.”
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. 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.
“A strong membership, combined with the strong secretariat that we have built up over the last few years, gives me plenty of confidence that the BSA is well placed to continue to play an influential role in the future.”