With the requirement of Life Cycle Assessments (LCAs) for all new construction activities from 2030 driven by the EPBD recast in the EU and SIA 390 in Switzerland, there is an urgent need for harmonized and scalable LCA data infrastructure, to support design decisions and policymaking for building and construction sector. This study presents the development of the first Swiss LCA data repository, compiling over 200 building LCAs into a structured data lake. The dual aim is to (1) identify technical challenges in aggregating heterogeneous building LCA datasets, and (2) explore the added value of such a repository for defining carbon thresholds and guiding low-carbon design. A pipeline was developed to harmonize diverse LCA reporting formats, normalizing results to kgCO₂-eq/m² of gross floor area and aligning system boundaries to EN 15978. Data extraction and cleaning involved large language models (LLMs), validation of units and ranges, ontology mapping to a unified element taxonomy, and handling of outliers and missing metadata. Structured JSON formats enable machine-readable import/export for future reuse and integration. Exploratory data analysis reveals consistent patterns: structural systems are the dominant contributors to embodied Global Warming Potential (GWP), followed by envelopes and building services, particularly in large multi-family buildings. Interiors and finishes, often underestimated, show significant influence in new construction. Sensitivity analyses highlight the impact of assumptions on service life and system boundaries, reinforcing the need for explicit metadata to ensure comparability. This work demonstrates that large-scale building LCA data aggregation is feasible and valuable for benchmarking, design decision-making, hotspot identification, and Environmental Product Declaration (EPD) prioritization. However, achieving this at scale requires a harmonized framework for reporting, scope definitions, metadata standards, and machine-readable formats supported by shared governance and technical maturity to ensure consistency, transparency, and policy relevance.