Saitech

Just found a fix mod for this problem added to the Steam Workshop by Griffinsong - It's called "Dawnguard Breezehome Alchemy Lab Fix" and it's under Category: Gameplay, Houses, Miscellaneous. Also, thanks to gsmanners for the console commands.

I am having this exact same issue since getting Dawnguard, I have done most of the standard stuff including making sure drivers and things are all up to date, re-installing Microsoft visual c++ and turning off what few basic mods i have and nothing seems to help at all. I even looked into trying to add Skyrim to the data execution prevention list but as I have my game via Steam I'm not sure that's possible.

I have this same issue since getting dawnguard, the only info / fix i have managed to find so far is to remove the dawnguard dlc, buy the alchemy lab and then add dawnguard again. I have not tested this fix myself yet as I am at least half way through the main quest line and I don't really want to risk removing dawnguard and possibly screwing up / losing something else in the process. For now I will just have to put up with not having a lab in that house.