If you choose to start a PhD without funding then you will need to plan in advance. You can use online sources to discover the average living costs of the city in which you intend to live. Adding to this you would need to consider rent, bills and potential travelling. This can give you a rough guide as to how much financial reserves you would need to save to get you through the 3 or 4 years of PhD. However, I would highly recommend researching what grants and scholarships are available from both the University and governing bodies. The sensible option would be to secure a grant or scholarship beforehand. However, if you feel ambitious then you could take a risk.
I had joined a PhD programme having saved enough to self-fund myself for the first year. Becoming an internal student could make you in a favourable position to receive university scholarships over external applicants, although this is highly based on the quality of your work, what type of scholarship it is and how much is in the pot. Some funding is set aside for specific research for which you may not fit the specifications. Work with your supervisors and departmental representative to understand the requirements. You can work towards fulfilling these requirements over the year to strengthen your application. Nevertheless always have a backup as this option could force you out of your programme resulting in a ‘wasted year’.
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