Nuclear metrology is an essential aspect to consider for further improvements of the initial characterization of sites under decommissioning. The H2020 EURATOM project INSIDER in June 2017 aims at improving the management of contaminated materials arising from Decommissioning and Dismantling (D&D) operations by proposing an integrated methodology for radiological characterization. This methodology is based on advanced statistical processing and modelling, coupled with adapted or innovative measurement methods. A metrological approach supports the qualification of this integrated methodology with a concrete application to real projects representative of the use cases identified in the project. Assessment of the outcomes will be used for providing recommendations and guidance resulting in pre-standardization texts.