It seems completely alien to me that a third party could claim ownership of something I've built because I'm using a craft / skill that they pay me to use too.

It seems completely alien to me that a third party could claim ownership of something I've built because I'm using a craft / skill that they pay me to use too. I live in the U.K. And work as a contractor. I've seen odd looking IP clauses in contracts but have amended them or felt confident I can sign them irrespective.