Installing ventilation fans are a great way to help reduce the moisture in our homes. Calling an electrical contractor is a great idea, yet you may require a roofer as well (when installed in an attic). Any electrical contractor that you speak with SHOULD be telling you that you may need a new line from the electrical panel. The Ontario electrical code states that we are allowed to have 12 devices (lights, plugs) on a circuit. So I would recommend simply turning off the breaker for the bathroom light and counting what went off… then you will know just as well as any electrical contractor if you need a circuit or not.
In the province of Ontario, any electrical work performed without a permit can result in fines up to $50,000.00. Therefore if somebody tells you “YOU DO NOT NEED A PERMIT” that is first person you kick off your list of potential electrical contractors. It’s not worth the risk to you and your family.