HB2014

Just hoping someone could help me understand a tax code change please? I was employed with one employer until the end of July with a tax code of 1275L, all pay through the PAYE system. I then stopped working for employer no. 1 and commenced with employer no. 2 in August. My tax code has been changed to K97X and when checking the calculation this appears to be based on a changed personal allowance of approximately 4850 and then deductions of approx 5750 due to an underpayment of tax this year of 1440. Im struggling to understand were these figures have come from (with the underpayment and reduced personal allowance initially) I will not earn over 100,000 this year, nor do I have any additional income. According to my online account, my tax for last year hasn’t been calculated either. Any help would be appreciated! Thanks in advance