The parent assembly "House" is fixed in place (shown by the (f) in the feature tree).
If this correct, and just DummyCarport should move, RMB on House & open Properties, then select "Solve as Flexible".
If House should move, RMB on it & select "Float".
I see now that my strategic mistake was inserting the dummycarport into the house sub assembly instead of the top assembly.
The logic there is that the carport is related to an external slab, not to the house slab, but I overlooked that, and had difficulty in moving the carport in order to improve the overall design.