I work in the Netherlands since January 2015 but just last month I had a permanent contract. According to my understanding I'm currently eligible for the 30% ruling according to the rules.
My employer prepared a letter to support my request. Can I still make the request to have 30% ruling or is it too late?
Whether you have a temporary contract or a permanent contract is irrelevant for the 30% ruling. So if you have a temporary contract (which is very common) you can apply for the 30% ruling.
It may be that you employer has some kind of policy that they will only support the application for the 30% ruling if the employee has a permanent contract. This is very unusual though. But the tax authorities won't make a difference.
You can still apply for the 30% ruling, even when you are already in the Netherlands for some time. The tax authorities will base their decision to grant you the 30% ruling or not on the situation the day you arrived in the Netherlands (and started working). If you met the requirements at that moment the ruling will be granted. A consequence of the late application is though that it won't be backdated till the startdate of the contract. It can only be granted from the first day of the month following the month in which the application was filed. The maximum period of 8 years for which the ruling can be granted will be reduced with the period you are already staying/working in the Netherlands. So if you are in the Netherlands already a year, the ruling can only be granted for the remaining 7 years.Expatax can help you with the application.