Paragraph 22 of the guidance:
"Secondments abroad that are an integral part of a job based in the UK are permitted by Work Permits (UK), however they will be considered as time outside of the UK for settlement eligibility purposes. To qualify for settlement in the UK you must have spent a continuous period of five years in the UK except for short holidays or business trips. Any secondment that requires you to be outside the UK for an unbroken period of over three months will make your previous stay in the UK ineligible as a continuous period in the UK."
So it won't affect an extension.
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