Welcome to the communities - it will depend a little on how your contracts are written but you could start with something like

"Work Location:
You can work from a location that suits your needs and the needs of our organisation whether that is from home, or OFFICE LOCATION. Your working location should be agreed in advance with your manager

Please note that there will be times when you are required to work elsewhere as and when we reasonably require."

This would work if its fluid - if you need to specify certain days in office and certain days at home then just change the first paragraph.

In reply to Keith:

That’s very helpful and useful, thank you