Accessing adult autism diagnostic pathways can be difficult. This study explored perspectives of UK autistic adults, relatives and clinicians regarding the characteristics of optimal adult autism assessment and diagnostic services. In stage 1, three key stakeholder groups were surveyed about experiences of adult autism diagnostic services (pre-assessment/assessment): 343 autistic adults, 45 relatives and 35 clinicians completed parallel surveys. Information from stage 1 surveys was used to devise statements for a modified Delphi process in stage 2 seeking consensus among clinicians on optimal diagnostic service characteristics. Data analyses were non-parametric and descriptive. Over half of adults were in contact with mental health services prior to autism diagnosis. Clinicians reported that multidisciplinary diagnostic teams lacked key professionals. Thirteen statements describing optimal autism diagnostic service provision were developed. There was consensus from clinicians on 11 statements relating to clear assessment pathways, updates for people while waiting, pre-assessment information gathering/provision, co-occurring condition identification and training/networking. Some autistic adults, relatives and clinicians were positive about services, all stakeholders identified improvements were needed. The findings describing optimal service provision are relevant for UK clinicians, managers and commissioners to improve diagnostic assessments for autistic adults, and have international relevance for similar health systems.