Seismic site characterization of rock and soil properties has a large impact on earthquake ground motions and engineering seismology, especially for evaluation of local site amplification, calibration of strong-motion records and realistic shaking estimates at seismic stations, site-specific hazard assessment, estimation of ground motion models and soil classification for building code applications. However, there is not yet a common way to exchange site characterization information, whereas setting-up standard practices and quality assessment are becoming very important to reach high-level metadata. Within the framework of the SERA “Seismology and Earthquake Engineering Research Infrastructure Alliance for Europe” Horizon 2020 Project, a networking activity is leading to the definition of a European strategy and standards for site characterization of seismic stations in Europe. Based on the results of an online questionnaire, we first defined a list of indicators considered as mandatory for a reliable site characterization: fundamental resonance frequency, shear-wave velocity profile (Vs), time-averaged Vs over the first 30 m, depth of seismological and engineering bedrock, surface geology, soil class. We then proposed a summary report for each indicator, containing the most significant background information of data acquisition and processing details, and a quality metrics scheme. This requires the evaluation of both (i) reliability of the site characterization indicators provided by different methods, and (ii) consistency among the indicators according to the current knowledge and experience of the scientific community. The quality metrics application to some Italian accelerometric sites, characterized within the Italian Civil Protection Department-INGV agreement (2016 to 2021), highlights the capabilities of capturing the characterization quality. These guidelines have been shared within European and worldwide scientific community and validated through focus groups during a dedicated workshop (https://sites.google.com/view/site-characterization-workshop/). They represent a first attempt to reach high-level metadata for site characterization, being aware that they can be improved and modified after a few years of experience and feedback from users.