Can you claim the startup costs of a business before incorporating?

These costs are capital costs. They are not revenue costs. The Revenue say clearly on their manuals page BIM35660 that when the course is to give new expertise, knowledge or skills to the proprietor then this is a capital expense. Similarly on page BIM35525 they explain that setting up a company is a capital expense. I am not sure exactly what you have in mind when you write 'office set up', but the definition of a capital expense is one made 'not only once and for all, but with a view to bringing into existence an asset or an advantage for the enduring benefit of the trade'. I would have thought that 'office set up' falls within this.