Accountacy Age comments here on an aspect of the Pre-Budget Report that I hadn't really considered until now.
The idea behind the PAYE system is fairly simple. Every employee has a tax-free personal allowance for the tax year. A code is issued to their employer that shows they are entitled to that allowance. An employee who is paid monthly will be given credit for one-twelfth of the allowance each month. That means that the right amount of tax is collected by the Exchequer, and the employee's net pay remains stable throughout the year.
The PAYE code can be adjusted to smooth out other tax adjustments too - say, tax due on a company car, or tax relief due on pension contributions, but the principle remains the same.
However, from 2010/11, the personal allowance will be tapered away for those earning more than £100,000. The exact personal allowance you get will depend upon your exact income for the year.
But people's earnings aren't always predictable. They change jobs, get bonuses, get and change company cars. That means that under this new system you often won't actually know what a person's personal allowance should be until after the year has finished! So it's impossible for the taxman to issue a PAYE coding notice that's accurate. PAYE is almost bound to get it wrong, and the difference will have to be sorted out via the self-assessment system. That means a) more paperwork and b) more unexpected tax bills for taxpayers.
But, as I've said so often before, this is what happens when legislation is rushed through - especially by people with little or no experience of the real business world.