If your employer doesn't offer any benefits to part-time workers, I don't see why you can't renegotiate your hourly rate when you switch to part time*. When you get hired, HR usually love to explain how much money you will actually make if you count all the benefits in. Just redo the same mental exercise again to find a salary estimate for a full-time employee without any benefits, then scale that number by the percentage of time you're willing to work.

(*) unless there's a law which gives you the opportunity to go back to the full-time while keeping your part-time hourly rate and getting all the benefits back, in which case the answer is "because your country has poor legislation for part-time workers".

If your employer doesn't offer any benefits to part-time workers, I don't see why you can't renegotiate your hourly rate when you switch to part time*. When you get hired, HR usually love to explain how much money you will actually make if you count all the benefits in. Just redo the same mental exercise again to find a salary estimate for a full-time employee without any benefits, then scale that number by the percentage of time you're willing to work.

(*) unless there's a law which gives you the opportunity to go back to the full-time while keeping your part-time hourly rate and getting all the benefits back, in which case the answer is "because your country has poor legislation regarding part-time workers".