The environment in which Vocational Education and Training (VET) schools operate changes rapidly due to more varied student populations, personalised, blended and work-based learning paths, developments in digitalisation, industry, and labour markets. These changes and policy pressures to ensure VET quality, demand teachers to continuously develop their professionalism and demands efficient and effective teacher professional development (PD). This research purpose is to establish design requirements (DR) for effective PD that are usable in different VET institutes and therefore suitable for larger scale implementation. As one of the two iterative circles in a design-based research project, this study is based on findings from the first iteration. The context, respondents and the PD program were comparable to the first iteration. Data were collected with a mix of methods using validated instruments in several levels of impact (attitudinal-, organisational support, reaction- and learning). A comparison of the results from both programs resulted in several DRs which appeared to be functional in both studies. The DRs: 'tolerance for complex contexts', 'contribution to a professional identity' and 'team based and collective participation' proved to be effective in two iterative cycles of testing and are therefore suitable for large(r) scale implementation.