My Contract has been extended and I now know that I will be onsite for 2 years. What do I need to do?

Expenses can only be claimed at temporary places of work. This is defined as less than 24 months, and at the point where you have been on site for 24 months (irrespective of whether it was through Simplify, or another Umbrella Company) you can no longer claim expenses. If you have worked on site for a year, and you receive a contract extension for 18 months, then at that point you are aware that the assignment will last longer than 24 months, so cannot continue to offset expenses.