You could argue that she is not entitled to any share of your deposit on the basis that that would be unjustly enriching her and that there was no intention to do so in the event of a separation. However, the amount of money you are talking about is substantial and my recommendation is that you see a solicitor and have a proper agreement drawn up, called a cohabitation agreement, clearly specifying what is to happen and who is to get what in the event of a separation. This would be binding and the courts would follow such an agreement in the event of your separation. It will cost about £500 and could be the best £500 you have ever spent if there is any risk of a separation whatsoever. Happy to discuss further. I hope that helps. Please leave a positive rating so that I am creditd for my time.