The implementation of green building is a good practice to mitigate environmental issues from the construction sector, especially in Indonesia which has experienced rapid infrastructure development. Since 2009, green building construction has been adopted in Indonesia, with performance evaluations based on the Greenship v.1.2 certification. This success has prompted the government to align the implementation of green buildings across Indonesia, particularly in the “Ibu Kota Negara” shortened to IKN. “Ibu Kota Negara” (IKN) will be inferred as Capital City Nusantara (CCN) in this paper. Capital City Nusantara (CCN) project through a more technical and updated regulation with the Ministry of Public Works and Public Housing of Republic of Indonesia Regulation, which is the Ministerial Regulation No. 21 of 2021. This study aims to evaluate the application of green buildings based on these two standards by Green Building Council Indonesia (GBCI) and by the Ministry of Public Works and Public Housing. This study also estimates the cost for its green building improvement through simulations to meet its desired target and the highest possible score. The methodology used is descriptive qualitative and quantitative analysis to assess the fulfilment of green building variables. The research object itself is Apartment Flat Buildings in East Kalimantan. The actual assessment results show that during the design phase (as the building is not yet built during this research in Early – Middle of 2024) the Apartment Flat Building achieved 49 points with a Gold rating according to Greenship v.1.2, and 127 points with a Madya rating based on Ministerial Regulation No. 21 of 2021. Several improvement simulations were conducted based on the technical recommendations from both standards to achieve the target rating.