My personal experience with this. I live in the US, for perspective.
As a person with autism, the Americans with Disabilities Act (ADA) does provide some limited legal protections, accommodations, and considerations within the legal and employment environments.
Within the context of the work environment, one must apply for accommodations. Within the corporate world, a person with autism who is requesting accommodations will have to apply through the Human Resources department. The company will have these forms posted on the corporate website OR Human Resources will send them to you. Part of the application REQUIRES a physician or psychologist to send Human Resources appropriate documentation describing "the condition(s)" one has been diagnosed with. In other words, you can't just say, "I have a specific condition and I am unable to perform these specific tasks,...or I would like to not do these specific tasks,...or I need special assistance aids",...whatever the case may be. If you are a "self diagnosed" autistic,...you're out of luck. Even a person with a broken arm or leg must fill out similar forms for accommodations within the workplace. Human Resources,...if they receive the proper paperwork,...and they interview you,...and they approve your request,...will then contact your manager to decide how best to approach the situation. Having said that,...probably the most difficult part is actually getting your doctor to fill out the paperwork in a timely manner,...my employer was surprisingly pretty cool and accommodating about the whole situation. However, I literally had to access my own medical/psychological records and print them off myself. So, be prepared to do that, in some cases. Furthermore, the ADA forms have a time limitation,...from this date, to that date,...they expire. So, if this is going to be an "on-going" situation, as is the case with autism, there is a possibility of having to refile paperwork,...I don't know,...talk to Human Resources about this. I had to have accommodations for a short period of time,...and don't at this time.
If you work for a small business, and you work directly under the owner/manager,...and there is no Human Resources department,...legally, the same process is in place,...but how the employer deals with you specifically, I suspect is highly individualized.