Interoperability (IOP) is the ability of a product or system – whose interfaces (APIs) are publicly documented – to connect to and operate with other products or systems, without restrictions. Interoperability further enables information and usable data to be properly exchanged and ensures the alignment of different business processes in critical sectors. In addition, is a prerequisite for transparent, domain-agnostic, and sustainable public sector digital services, where Public Administrations (PA) can efficiently interact across borders and domains by using common frameworks, standards, and processes for sharing information and data. The European Interoperability Framework (EIF) enables interoperability with guidelines for digital services. Therefore, the alignment with EIF becomes pivotal for the European Union (EU) countries since different regulations that facilitate and impose the implementation of European policies such as the Single Digital Gateway (SDG) regulation and the Once-Only Principle (OOP) consider the IOP a crucial technical and operational component for government digital services. This article proposes the update of the Greek NIF, with guidelines of EIF, OOP and other technological trends in conjunction with new legal and policy provisions. This proposed assessment methodology can be reused in other countries and can be further adapted for updating the EIF. •Interoperability is in digital services and is a prerequisite for transparent, domain agnostic and sustainable public sector digital services.•This proposal contributes to digital transformation and interoperability knowledge by providing a systemic approach methodology for the identification of gaps and the proposal of concrete improvements and recommendations.•The methodology includes the conceptualized technological trends such as the Once Only Principle (OOP), the CEF Building Blocks (BB) and the Single Digital Gateway (SDG) regulation, for this proposal, in conjunction with new legal and policy provisions and can be reused in other countries and for the update of EIF.