Good design is longlasting
It’s at least fair to say that the UI design of many council websites lasts a long time, even if the reason for that is because staff resources to spend time refreshing the design every year or every couple of years have been slashed over the last 10 years or so.
A longlasting Service Design will be a simple and modular Service Design. The steps necessary to complete an end to end service delivery process – and the accompanying design documentation - will be reduced to as few steps as possible, with as few decisions as possible. Where there are many decisions and many steps which genuinely are necessary in the Service Design, try to see if a modular approach is possible, where you isolate design components to minimise the impact of changing one part of the design on other parts of the design. Similarly modular and sandboxed UX and UI Designs can also help you to create a longlasting overall design whilst leaving you free to update the modules according to changing needs.
In short, you can’t anticipate what exactly you might need to change from year to year, or even at times from month to month, but you can design in such a way that the impact of change is mitigated.