Complex sentence: In 2012, the Duchess announced her plan to finish reconstruction of The Alnwick Garden by May 2015 and to step down from managing it the same year, when the visitor attraction will be franchised out to an external management company.
Split into simpler sentences: In 2012, the Duchess announced her plan to finish reconstruction of The Alnwick Garden by May 2015. She has made arrangements enabling her then to step down from managing it and for management of the visitor attraction to be taken on by an external management company.