The ethnomathematics module aims to help students better understand and appreciate mathematics daily by combining mathematical concepts with regional cultural backgrounds. This research aims to develop a valid and effective ethnomathematics-based module on flat building materials for class II SD N Depok 2 Sleman Regency. This research is development research (R&D) using the ADDIE development model. The object of this research is an ethnomathematics-based module on plane material. Indonesia is a country that is rich in culture and has excellent potential in the application of ethnomathematics. Various ethnic groups in Indonesia have developed unique mathematical practices in their cultural activities, such as traditional buildings and food. Ethnomathematics can be essential in strengthening local culture through an independent learning curriculum. This learning combines mathematical concepts with regional culture. Based on the results of observations made, it was concluded that the learning process at SD N Depok 2 Sleman Regency was still not optimal because, in the learning process, they only used textbooks from the government, which were the only source of learning during learning. The subjects of this research were class II students at SD Negeri Depok 2 Sleman Regency, totalling 28 students, one lecturer, and one teacher, who were the validators. The data analysis techniques used are qualitative and quantitative data analysis. The results show that this research produced an Ethnomathematics-Based Module on Flat Building Material for Class II, which is feasible based on expert validation results with a score of 91%, and student response results are very effective with an average field trial score of 92%. Based on these results, developing an ethnomathematics-based module on flat building materials is feasible and effective for the phase I independent curriculum mathematics learning process.