Design work is billable work.
Often, when a commissioner asks you for a drawing they simply throw ideas around and expect you to come up with something cool, either for an environment/background, a costume, a vehicle or a creature, wihtout giving you any details about it. This, of course, requires you to design it.
Which is fine, until they refuse to pay extra for the time you will invest in this design work.
Designing anything takes considerable mental effort. It is, truly, where the 'creative' part of any skilled profession comes into play, and artwork is no different. Some would say it's even harder becuase there are no rules: In a drawing literally anything goes, and there are no bothersome laws of physics or budget constraints to keep your lofty ideas tethered. Given this, it is astounding that a commissioner who doesn't give you reference for what you are creating would be shocked and even upset when you present them a bill that includes design time.
Maybe my background in Engineering has gotten me used to being very careful with how I bill my time, and that's why when I open for commissions I explicitly state that either commissioners have to give me reference for what they're asking me to draw (visual reference of previously designed characters, vehicles or environments), or they will have to pay for the hours it takes me to design it. Your work as an artist is valuable, the time you spend designing something often being the most valuable of all.
Mind you, this only relates to the design work. Ownership of the rights of what I design for pay is a different topic I'll talk about in a future post.