It won't meet any building code: Electrical / plumbing / insulation / windows / doors / you would have to be zoned for R2.

Depends on where you are, demand, current structural issues, insurance and maybe local codes.

500 is enough space for a room+bath+kitchenette. Is it appropriately built and insulated? Working and legal egress windows?

Is there demand in your area? Are you in a location where someone would pay to live there? Will you need to provide them with parking space or will they walk/bike everywhere?

Legally you cannot do detached ADU's in our city. About 10% of people do anyway and some are smaller than what you have. All have appropriate insurance. If they don't have appropriate off-street parking then they advertise as 'walk/bike only' which is fine as we have good protected bikeways everywhere so renters can easily walk/bike to local stores and to transit (we're a suburb so the 11 mile trip to the city is more than most want to do and it does not have good protected bikeways the entire way).

Owners largely do a good job of being good neighbors so we've had no problems with noise or such. If there is a problem then someone talks to the owner who takes care of it. One new homeowner did report their neighbor's ADU (for being an ADU, not for any actual problems) to the city and from what I heard the gal at the city kept telling him that she couldn't understand what he was saying. While many cities enforce ADU regulations, ours is more of a just in case - if there's a problem then they have legal grounds to deal with it but so long as there are no actual problems then it's not a problem.

Worth it? Financially it depends on how much you can charge less costs for maintenance and insurance. OTOH, many people have become quite good friends with their renters.