Young adults in the Global North occupy a contradictory environmental identity: they are purportedly more environmentally concerned than older generations, but are also labelled hedonistic consumers. Most studies have focused on young adults still residing in parental homes, neglecting that Generation Y (born between 1975 and 1991) has 'grown up'. The consumption patterns and environmental implications of their newly established households demand scholarly attention. Through a large-scale household sustainability survey, conducted in Australia, we have uncovered important inter-generational differences in environmental attitudes and everyday domestic practices. We found that generational cohorts hold distinct environmental attitudes. Younger households were most concerned with climate change, and least optimistic about future mitigation. However, generational differences influenced everyday domestic practices in more complex ways. All households engaged extensively with those 'pro-environmental' practices that reflected established cultural norms, government regulations and residential urban form. For other pro-environmental practices there were clear differences, with Generation Y households being the least engaged. A widening 'value-action gap' was apparent across our sample population, from oldest to youngest. However, rather than reflecting Generation Y's supposed hedonism, we argue that this gap reflects generational geographies: how lifecourse intersects with housing and labour markets and norms of cleanliness to shape everyday domestic practices. Our research illuminates the shortcomings of a one-size-fits-all approach to household sustainability. The young adult stage is a time of transition during which homes and independent lifestyles are established, and practices are altered or become entrenched, for better or worse.