In the Supergirl episode Welcome Back, Kara!, Supergirl explicitly states that the Fortress is the same one Kal-el uses, but from all visual indications it's clearly not. Her version is more decorated, more open and spacious, and prominently displays Kryptonian technology, whereas the set they use on Superman and Lois is quite sparse by comparison and has a similar but distinctly different "look-and-feel" to it.
It seems like it has to be intentional; if both shows are being made by the same studio, (which they are, and both are filmed in Vancouver, BC to boot,) it would not only improve consistency to re-use the established set, it would save money too! But they went and built an entirely new one for Superman and Lois.
Do we have any explanation for this?