Background: Recent recommendations promote the inclusion of people living with dementia beyond the role of 'participant' to involvement in all areas of the research process. This reflects shifts in dementia studies from 'research on' to 'research with' people living with the condition. In this paper, we describe the design process and features of a modified Delphi survey devised through consultation with people living with dementia. Methods: This article focusses on consultation with people living with dementia and care partners to design an accessible Delphi survey to facilitate participation in core outcome set development. We used the COINED model of co-research developed through the ESRC/NIHR Neighbourhoods and Dementia Study to structure consultation on three features of modified Delphi design. Consultation was achieved through 1:1 and group sessions with a total of 28 individuals (18 people living with dementia and seven care partners). Results: A flexible, responsive and adaptive approach to ongoing consultation with people living with dementia and care partners through 1:1 face-to-face sessions facilitated: (1) the development of a 3-point non-categorical importance scale; (2) the translation of 54 outcome areas into 'accessible statements' for a two-round Delphi survey administered to five stakeholder groups (people living with dementia, care partners, health and social care professionals, policy-makers and researchers); and (3) the delivery of a Delphi survey. These features of core outcome set development facilitated the involvement of people living with dementia in study design and as research participants in the data collection phase. Conclusions: Involvement of people living with dementia as a key stakeholder group is not reflected in studies using Delphi survey methods for core outcome set development. Time, resources, researcher expertise and support, underpinned through targeted funding facilitate meaningful and productive inclusive approaches, now an expectation of dementia research. Trial registration: The study is registered on the COMET Initiative.