In low- and middle-income countries, poor autonomy prevents women from making financial decisions, which may impact their access to improved sanitation facilities. Inadequate access to improved sanitation disproportionately affects women's and children's health and wellbeing. Although socio-cultural factors are known contributors to gender inequity, social beliefs that potentially motivate or dissuade women from making sanitation-related household decisions are not well understood. These beliefs may vary across settlement types. To empower more women to make sanitation-related decisions, the relevant socio-cultural norms and underlying social beliefs need to be addressed. In this mixed methods study, we explored women's role in sanitation-related decision making in three settlement types, urban slums, peri-urban, and rural communities in Bihar. Trained qualitative researchers conducted six focus group discussions with women of two age groups: 18-30 years old, and 45-65 years old to understand the norm-focused factors around women's role in getting a toilet for their household. Using insights generated from these group discussions, we developed and conducted a theory-driven survey in 2528 randomly selected participants, to assess the social beliefs regarding women making toilet construction decisions in these communities. Overall, 45% of the respondents reported making joint decisions to build toilets that involved both men and women household members. More women exclusively led this decision-making process in peri-urban (26%) and rural areas (35%) compared to urban slums (12%). Social beliefs that men commonly led household decisions to build toilets were negatively associated with women's participation in decision making in urban slums (adjusted prevalence ratio, aPR: 0.53, 95% CI: 0.42, 0.68). Qualitative insights highlighted normative expectations to take joint decisions with elders, especially in joint family settings. Surrounding norms that limited women's physical mobility and access to peers undermined their confidence in making large financial decisions involved in toilet construction. Women were more likely to be involved in sanitation decisions in peri-urban and rural contexts. Women's involvement in such decisions was perceived as widely acceptable. This highlights the opportunity to increase women's participation in sanitation decision making, particularly in urban contexts. As more women get involved in decisions to build toilets, highlighting this norm may encourage gender-equitable engagement in sanitation-related decisions in low-resource settings.