At the moment, all passport holders from the USA, Canada and the UK must be accompanied by a licensed guide for all travels in Iran.
This essentially means that unless you hire your own private guide for your entire duration of your visit, you must join an organized tour.
Citizens from these countries may be able to apply for a visa on arrival, as long as they have an authorized letter saying they will be on tour, and their guide is waiting for them in the airport.
However, we strongly suggest you have your visa already before travelling to Iran. There were some tourists, waiting over three hours for their visa on arrival and occasionally they are from USA, and when you ask for the reason you find out that their delay is just because of the confirmation required from their tour company to confirm their trip.
IMPORTANT – If you are from the USA, Canada or the UK you must be on an organized, licensed tour to visit Iran, and they will assist with your visa application.