BIM-Specific Language-Model Pipelines for IFC Code Generation and Building Component Question Answering: A Full-Corpus Comparison of Zero-Shot, RAG, and Supervised Fine-Tuning

Natural-language access to Building Information Modeling (BIM) requires two complementary capabilities: generating structurally correct Industry Foundation Classes (IFC) code and answering questions about building entities, properties, constraints, materials, and interoperability workflows. This study compares zero-shot generation, retrieval-augmented generation (RAG), and supervised fine-tuning on the 42,680-record IFC-BIM High-Quality Alpaca corpus. A deterministic family-stratified 80/10/10 split assigned every record to training, validation, or testing. The corpus comprised 28,513 unique 36-statement IfcWall DATA fragments and 14,167 question-answer records distributed across 14 observed families. All three approaches used the same task-specific output constraints, so the comparison isolates the source of adaptation rather than changes in grammar. On 2,852 held-out code instructions, zero-shot generation achieved 0.18% pass@1 and 54.97% mean field accuracy. Top-1 RAG reached 68.62% pass@1 and 95.51% field accuracy, while the fine-tuned generator reached 100% on both measures. RAG errors were limited to the non-literal LoadBearing and IsExternal Boolean fields; majority voting over five retrieved neighbors increased pass@1 to 86.01%. On 1,417 held-out component questions, RAG produced the highest token F1 (85.19%), ROUGE-L (83.52%), and formal-constraint expression accuracy (52.38%). Fine-tuning produced the highest structured field accuracy (93.77%), exact match (60.83%), and IFC-term precision (89.13%). The findings show that supervised adaptation is decisive for regular closed-schema code generation, whereas retrieval remains valuable for detailed explanations and rare symbolic expressions. A practical BIM assistant should combine supervised routing, evidence retrieval, schema-constrained decoding, and executable validation rather than applying one adaptation mechanism to every task.