CPV code 45212321 designates Auditorium construction work in the Common Procurement Vocabulary (CPV 2008), the EU classification used to describe the subject of public contracts.
45212321 is a leaf code — the most specific level of its branch — so it can be used directly as the main CPV code of a contract notice. If it does not quite fit, check the neighbouring codes under 45212320 (Construction work for buildings relating to artistic performances).