Generally speaking (although the details of probate vary substantially from state to state, while the basic tenets remain the same, but you'd need to double-check the relevant state law), a will provision that requires a criminal act (arson) is unenforcable and it would be fairly simple to get a judge's ruling to that effect. I believe, although it's not really within my expertise (I have probate law experience, not criminal law experience), that arson includes torching one's own building, even if it's not a fraudulent act (i.e., to collect on insurance), b/c of the risk to firefighters and abutting property owners' assets.
The only legal way I can think of to burn down a building is to arrange for the local fire department to do a controlled burn for practice and training, depending on whether there are any hazardous substances (e.g., lead paint, which will be in most houses built before 1970 or so, or asbestos) in it. If so, the hazardous substances would have to be removed first. And the house would have to be distant from other buildings, etc., so as to avoid any other risks, and meet whatever other safety requirements they have. I'm not even sure a fire department would agree to do such a thing, b/c that's outside my expertise.
I don't know why it has to be a fire, unless it's to destroy some evidence in the house. If it's just to eradicate a place that the testator has bad memories of, it would be more credible to require that the building be demolished. And then you'd have to address the question of why the testator didn't do it himself during his lifetime, which is also true of the fire -- why didn't the testator arrange it?
Come to think of it, there's also a legal concept -- the technical name of which eludes me -- that criminalizes wanton destruction, even if it's of one's own property, so not only arson but also demolition might be a criminal act, depending on the circumstances. If the house is decrepit, requiring that it be demolished would be legal, but if it were in mint condition, there might a problem with this concept. If I can think of the term later, I'll post it.
JD, not giving individual legal advice, just general information.