Martin Moszkowicz, executive producer of The Mortal Instruments, has denied the rumor that City of Ashes and City of Glass will be filmed back- to- back in 2013. According to him, City of Ashes will be filming later this year with Harald Zwart returning as director.
Back to back filming has been mentioned many times. It is used as a means to save production costs but it sounds like Constantin Film is going to work on one film at a time. It makes me wonder if they are wondering how City of Bones will do in theaters before committing to two (or hopefully more) sequels. This could be the decision of the production company or director, Harald Zwart or as TMI Source pointed out, City of Glass could actually begin filming in 2014, which is technically “next year”.
— Martin Moszkowicz (@mmoszkowicz) June 13, 2013