So, an extraordinary amount of rules in 5e are left up to my interpretation. I’m going to attempt to organize them into this wiki of House Rules.
Passive checks vs. Time Sensitive checks
The d20 roll is meant to incorporate the element of random chance when performing an action without the luxury of having all the time in the world to accomplish it—this is known as the combat timeline: climbing a broken wall while taking arrow fire, recalling pertinent information about a dragon while locked in combat with it, picking a lock while the room fills up with water, etc. All other times (also known as the narrative timeline), you will use the passive result to accomplish a task:
10 + (Attribute Bonus) + Proficiency Bonus (if applicable)
This represents not having any distractions while still performing the task with the same amount of time. If a player wishes to assist another player, then they may use a 15 in the above formula instead. If time is no object, than a player may take twenty times the normal time it takes to perform the action and use a 20 in the above formula. Performing an action this way is a one man job, however, and no one may assist him—unless they search different parts of the house, etc.
Perception and Investigation are rolled by the DM in secret
Now, what I’m attempting to do is to make it easier to succeed at things when in narrative time (use passive checks or take a long time to roll a 20 on the check), so that the party has to make the choice of having the best chance to find all the hidden goodies, or save time. For time sensitive checks, however, the party may elect to spend in time in combat rounds to perform actions and be told “yes, you found a trap”, or “you find nothing”, etc. The choice here becomes how many attempts and how many rounds do I want to spend looking for something while another significant event is occurring?
Considering Spell-Based Arson
The nature of magical fire is not really revealed anywhere in the books—we do not know how hot it is, what physical properties it has (flame, plasma, etc.), or why it doesn’t ignite obviously flammable material as long as it’s being carried/worn by someone. So this particular ruling will be primarily for balance’s sake:
The level of the spell will determine how large of an object/structure it will ignite in the combat timeline.
As far as narrative timeline goes, any spell (even cantrips) can be used repeatedly over time to commit arson.
I am working on the specific table for spell levels and object/structure sizes, but I will post it as soon as I have it.