Skilled worker visa extension - dependants

Hi there,

I am on Skilled Worker visa that was extended in summer till Dec 2026. The dependant visa of my family (partner and 2 daughters) is expiring on 22 Nov 2024. I have just applied for my partner's visa yesterday.

I was wondering if the visa extension application for my daughters could be delayed by few weeks, or is it mandatory for the whole family to apply near to each other.

Secondly, the visa extension for dependants is for 2 years, however, I was charged the IHS for 2.5 years for my partner's application yesterday. Is there any reason for this?

Thanks