ILC Compatibility with Possible Multi-TeV Upgrade

A large number of critical design decisions for ILC will have to be taken in the near future. Some of these decisions, especially those related to civil engineering are strongly related to the capability of reusing the ILC tunnel for a future multi-TeV upgrade based on CLIC technology. This note discusses the key issues concerning multi-TeV upgrade capability in order to assure that ILC decisions related to these issues are taken with the knowledge of resulting consequences. In particular we address crossing angle, tunnel curvature, ground motion, space requirements and constraints for technical services and electrical networks.