The growing complexity and numerous changes in Dutch vocational education and training (VET) pressures teachers to continually develop their teaching profession. Therefore professional development (PD) is needed, but policy-initiated and imposed PD might be ineffective. Thus the purpose of this research is to evaluate which design requirements (DR) in a mandatory PD programme delivered in a VET institute appear to show the most effectiveness. Based on a review study aimed at detecting effectiveness enhancing DR an assessor PD programme was redesigned and implemented. This programme was evaluated with respect to effectiveness on five different levels of impact considering participants' attitudes, their reactions, what they had learned or what behaviour they developed, and organizational support. Each level of impact was evaluated using existing and self-constructed instruments including questionnaires, surveys, interviews and observations. Thirty-nine teachers took part in this redesigned PD programme. Several DR appeared to show measurable levels of effectiveness: tolerance for complex contexts, contribution to a professional identity, and team based and collective participation.