This document describes the development and the integral parts of the BIGG harmonization layer. The harmonization layer is a core component in the BIGG Reference Architecture Framework that ensures the interoperability with external data sources, defines the data structure of the internal databases, enables the integration between the different components, and provides a harmonised data input to the BIGG AI Toolbox. The final harmonization layer consists of the BIGG ontology serving as a common reference, a Harmonizer tool enabling automated data harmonization of heterogeneous data sources over the BIGG ontology, and includes mapping and transformations for the different data sources used in the project. The process of development of the harmonization layer consisted of several steps. The first step was the development of the BIGG Standard Data Model 4 Buildings based on detailed analysis of requirements of the BIGG use cases, and of the available datasets from the pilots necessary for the execution. The data model set the semantic base and structure of data in BIGG and served as a common reference for the parallel work in the work packages of the project dealing with communication, data analytics, and integration of components in the reference architecture. In a next step, the Standard Data Model 4 Buildings step was transformed into a W3C standards compliant BIGG Ontology based on the RDF specifications, thus enabling the use of semantic technologies and machine understanding of data. The adoption of the RDF as internal format for representing data in BIGG enabled the development of the Harmonizer component, a generic tool for converting both static and dynamic building-related data into BIGG-compliant data in RDF. The Harmonizer allows to use RML mapping rules to align the input data to the BIGG ontology and also implements SPARQL queries to specify correspondence between standard ontologies (e.g., ifcOWL) and the BIGG ontology. Finally, aiming to contribute to standardisation, the BIGG Ontology was substantially analysed and compared to existing standards. This led to the development of BIGGstd, a standards-based transformation of the BIGG Ontology obtained by reusing existing standards. The transformation process served to evidence gaps in existing ontologies and to pinpoint potential contributions to them. A set of specific terms defined by BIGG for which no equivalent terms in existing standards were found was identified and presented in Annex 1. The potential use of these terms for future extensions of existing data standards will be considered by the BIGG representatives in EU standardization committees. In addition to the overview of the harmonization layer components above, the document provides links to their full documentation uploaded in the public GitHub repository of the project.