Minecraft / testforblock with radius?

I'm playing on the MC version 1.8.9 and would like to build a fire alarm with commands, because my friend often has problems with it, and his houses burn down almost completely. So I looked for a suitable command, but to my knowledge the / testforblock can only check one block. Could you tell me a way how it works, or if necessary tell me another command that does that?

/ gamerule doFireTick false

Then the fire will not spread and can't destroy the entire house.

Yes, I know him, but he wants it partly because he also wants to do slash-and-burn

Then he could build the block area of his house into a / fill command that replaces every fire block with air from a command block.

It would look like this: / fill x¹y¹z¹ x²y²z² minecraft: air replace minecraft: fire

So thanks for your help, but actually I'm looking for a fire alarm instead of a fire extinguisher. But thanks anyway