Do you need the king’s permission to charter a new city, or is your Lord Paramount good enough? If the former, is a Warden capable of chartering a city in the name of the king?

It’s hard to say, because the Darklyns were Crownlanders, so the Targaryens were their liege lords in either case, and we don’t have any examples of Lords Paramounts giving city charters. 

Certainly, we know kings can create cities – Aegon founded King’s Landing, Aerys II proposed creating new cities, etc. Whether it’s a concurrent power, I don’t know.

But I don’t think Wardens can. Wardens are military leaders, so their powers would be military in nature, as opposed to civil.