It's pretty common in the US (don't know about UK or EU). Whenever I'm starting up a side project, I generally ask for a written waiver covering the scope of t

It's pretty common in the US (don't know about UK or EU). Whenever I'm starting up a side project, I generally ask for a written waiver covering the scope of the project, signed by someone with sufficient authority (usually officer of the company, though different companies have different policies). That's important, because your boss may not have authority to commit the company to such things. Ask HR who does have that authority. IANAL, but in my experience being told verbally "that's cool" means precisely nothing when the company decides it's not cool. Having something in writing changes that.