You still need the correct approvals, planning permit and building permits, the same as any other structure.
Once the above is done and the flag pole is erected, you can fly a national flag, your coat of arms, or a Jolly Roger, or something else.
I had a disagreement with the council about some tree prunings that were on a garden bed in my front yard, they said it looked untidy. After a heated phone discussion, I said I would cover it with a tarp, they said okay, and I covered the tree prunings with a cheap bright blue tarp, (the council have left me alone since then).
