With software systems permeating our lives, we are entitled to expect that such systems are secure-by-design, and that such security endures throughout the use of these systems and their subsequent evolution. During my PhD, I aim to engineer sustainable adaptive security solutions that reflect such enduring protection in the dynamically changing security theatre of cyber-physical systems. I have chosen the example of a smart home as a cyber-physical system to motivate & illustrate sustainable adaptive security, discuss challenges for sustainably secure systems, and my research plan for engineering them.