PAYE Change Tax Code and Tax Deduction Increase

Hi, My annual salary increase from May and found that my tax code changed from 1257L to K683X, the Tax Deduction amount increase a lot Check HMRC that there is an Other Earnings items in PAYE section, however, I submitted the Other Earnings in Self Assessment section for 2023-2024, is it correct include in PAYE? To me looks like double charge for PAYE and Self Assessment tax payment? It also found that HMRC estimate I have underpaid tax around £300, but the Tax Deduction amount increase more £500, I dont know how to calculate this, does this amount will still charge the same amount in future? Thanks in advance

Hi Alan Chan,
When an individaul has a PAYE record and they also complete a Self Assessment tax return then following the submission of a Self Assessment tax return there is a process that takes place to automatically code certain items from the tax return and include them in the current years tax code.
Auto coding from a Self Assessment tax return takes into account certain entries made on the tax return for the previous year and can include casual earinings/other income included on the tax return.
It may also take into account any effect your current estimated pay for the current year held on the record has.
An IYA (In Year Adjustment) may be calculated to attempt to collect the estimated tax underpaid for the current year where possilbe during the year. The IYA will be the deduction needed to collect the anticipated underpaid tax for the current year at the relevant tax rate so the deduction for the IYA is not the same as the source amount/amount underpaid.
If you have concerns about your tax code for the current year we would suggest you contact us to discuss.
Income Tax: general enquiries
Thank you.