HMRC allow tax relief on 25% of your pension pot in a single pension scheme. If the lump sum in your DB pension is less then 25% of the total, the point of an AVC is that it’s in the same scheme, so you can use the rest of the 25%. Another benefit, if you’re paying 40% tax on any of your income, is that you can use the AVC to put enough in the pension to get your taxable income below the threshold.
From andrewreay’s link, it looks as if the main difference between shared cost and ordinary AVCs is that you can save NIC as well with a shared cost.
You have to stay within the Annual Allowance for how much you can put into a pension per year, since if you exceed that you don’t get the tax relief. What you’re able to put in an AVC is the Annual Allowance less the increase in value of your DB pension over the year. You can also carry forward any unused Annual Allowance in the previous 3 years. I started putting money in my AVC 3 years before I retired; in my case it was well worth the time spent understanding the rules.
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