When starting a new job, you need to indicate to your employer what your tax code should be by completing an IR330 form. If your circumstances change during employment, you need to inform your employer and complete another IR330 form. Tip: The most common cause of a tax bill is when an individual does not notify their employer when their circumstances change, resulting in the tax code being incorrect for a period of time. If you begin receiving another source of income, have a significant change in your earnings or your family begins receiving tax credits, check again that your tax code is still correct!