Child Benefit charge and earning over £100k

If your personal allowance is already reduced due to tax owed in a previous tax year, for example due to paying the high earners child benefit charge and you earn over £100,000 - will the already reduced personal allowance reduce at the same rate and be gone before you hit £125,000 of earnings?

I don't think your Personal Allowance can ever be reduced due to tax owed in a previous tax year. My guess is your PAYE tax code has been reduced. The misunderstanding probably comes from the fact that for many people, their PAYE tax code is made up of the Personal Allowance only - but in fact, the PAYE tax code is an overall summary of every part of your tax affairs that needs to be taken into account to deduct the right amount of tax from pay. In summary then, the reduction of the Personal Allowance due to earnings over £100k, and the additional earnings deduction to repay tax owed in a previous year, will both operate independently. This means that the combined tax free amount for PAYE (considering both Personal Allowance and tax owed) can go NEGATIVE as total earnings approach £125,140. There is another detail to be aware of, too: the PAYE tax code system by itself does not automatically adapt to Personal Allowance reduction over £100k. In the year a taxpayer receives a pay rise that moves them over £100k, or further towards £125,140, unless HMRC are notified of the increase in the full year's predicted earnings during the year, resulting in a changes tax code, PAYE will under-deduct throughout the year, leaving a balance to be paid at Self Assessment. In subsequent years, HMRC's earnings estimate for that employment will be updated based on the previous year, so it will start to be coded in to your tax code.