This may be related to Dawnguard, as it never happened to be before installing it. Anyway, I can buy all the decorations for Breezehome but the alchemy lab never shows up, Proventus was more than happy to take my money for it.. twice, but the spot in the house remains as bare as ever!

I'd love to know if there's a console code to force it to show up, I've paid for it twice after all. Any other method of getting it to appear would be greatly appreciated as well, thanks!

I have this exact same issue and it looks like Xbox 360 users also have this problem. They say that the only workaround they have is to save, uninstall dawnguard, buy the lab and save, re-install dawn guard, but apparently this runs the risk of loosing any dawn guard related gear, shouts etc. I'm going to make a backup of my save gave and try loading up skyrim with dawn guard disabled and see what happens,