CGT is charged on profits when certain assets are sold or transferred (1)
Everyone, including children, have an annual CGT tax free allowance, which is £11,700 in the 18-19 tax year.
So, what this means is that any gains realised for this amount or under should not incur any tax.
The Gains over this allowance are charged at 10% or 20% (2) dependant on any other income you may be in receipt of so reducing your taxable income in set years can help reduce your tax.
Your allowance can not be carried forward or back, but this is yours as an individual so a married couple can potentially realise a gain of £23,400 without incurring a tax liability
You can also offset gains and losses against each other if they were realised in the same year. You can also carry forward your losses and offset them against future gains providing you have registered the losses with HMRC.
1 You can transfer to a spouse or civil partner without incurring CGT
2 Gains from the sale of additional residential property are subject to CGT of 18% or 28%.