I've NC as this is all very personal.
I'll start with saying I don't currently live in the UK so benefits/ tax while very similar where I am are different.
I currently work 3 days per week, I receive social welfare along with this and also pay nothing in childcare due to being under the gov threshold.
My employer would like me to take on further responsibilities and work an extra day, she didn't put pressure on me and said she wanted to run it by me before looking for someone else externally. Her words 'she would love to have me FT, but this new role is also 3 days so she's thinking I'll do 4 days and we can take on a junior for 2 days'
Money - I am currently on 24k, so 8k per day. In order for this to financially be worth my while I would need to ask for 40k, so 16k for 1 extra day. I will lose all social welfare (naturally) and will lose most of my gov allowance for childcare, for either increase.
If I was put onto 32k I would only be €64 better off per month. 40k has me €418 better off per month.
It seems like a huge amount to ask for for 1 extra day. We only spoke about it this morning, she said to think about it and that I have first refusal.
YABU - don't ask for 40k, they will think you are a CF
YANBU - ask for 40k, no point in doing it if it's not worth while