INTRODUCTION. While diagnosis rates of autism spectrum disorders (ASD) and attention deficit hyperactivity disorder (ADHD) vary within countries at a large-scale municipal level, small neighbourhood geographic variation remains understudied. In this nationwide study, we describe the rates of ASD and ADHD diagnoses in children and adults by geographical data zones of approximately 2,500 residents across Denmark. METHODS. We included a population of children born from 1993 through 2020 and an adult population born from 1977 through 2003. We followed children from their first birthday and adults from their 18th birthday to either diagnosis, death, emigration or 31 December 2021, whichever came first. Data were analysed using multilevel log-linear Poisson regression adjusting for age and sex. Data zones, a data-driven approach to define small geographical neighbourhoods, were used as the unit for spatial analyses. We present incidence rates in data zones and median incidence rate ratios (MRRs) as estimates of the variation in rates of the disorders between data zones. RESULTS. ASD and ADHD diagnoses among children showed considerable variations between data zones (ASD: MRR = 1.44; 95% confidence interval (CI): 1.42-1.47, ADHD: MRR = 1.38; 95% CI: 1.36-1.40), suggesting that the incidence can be 44% and 38% higher in high incidence zones than in others. Similar variations were observed for diagnoses among adults (ASD: MRR = 1.44; 95% CI: 1.40-1.48, ADHD: MRR = 1.44; 95% CI: 1.41-1.46). CONCLUSIONS. The large variations might reflect differential treatment seeking, referral practice and diagnostic procedures across Denmark. FUNDING. This study received funding from BERTHA – the Danish Big Data Centre for Environment and Health, and the Novo Nordisk Foundation Challenge Programme (grant NNF17OC0027864). TRIAL REGISTRATION. Not relevant.