Transitioning from a JISA to an ISA

[againstabrickwall]

If a JISA that has reached its annual deposit allowance of £9k transitions into an adult ISA on a young person's 18th birthday, does it mean an additional £11k can be added before the end of the tax year, or an additional £20k?

I'm assuming £11k, but can't find a definitive answer online.

Good question. Watching for any replies as wondering myself

You used to be able to use both full allowances - JISA and adult cash ISA - for 16 and 17 year olds. It was an anomaly and I‘m not sure it wasn‘t abolished in some small print.

This answer from .gov.uk is as clear as mud: https://community.hmrc.gov.uk/customerforums/pt/96126d23-93aa-ee11-a81c-0022481b920a

This info from AJ Bell is more helpful, if it's right, though you have to click 'yes' to being a finance professional to read it : https://www.investcentre.co.uk/articles/maximising-tax-wrapper-allowances-children

It goes further than my question and says that they can have an adult isa from age 16 with its own 20k allowance, as well as their JISA.

I guess the new Government might make changes though.