So one of my players just texted me and asked me, "Hey, how would you deal with me burning down a town"?

Honestly, I have no idea how to go about handling it during a game. And while I don't like the idea of it, I don't tell my players no. The player in question wishes to burn down a major city of an area I made up myself. Let us assume the entire city is to go up in flames. I understand this is a very broad question and is hard to answer.

I need help mechanizing how the fire plays out.

How do I deal with structure damage whether buildings are saved or completely burnt down?