I have a lvl 24 high elf female and I have the Dawnguard dlc and since then I have noticed I can't buy the lab for my house in whiterun. I have tried to reload to another earlier save and I have also loaded and play my higher level ( 43 wood elf ) and then loaded back my high elf and that didn't work.

i believe you have to remove the Dawnguard dlc, buy the alchemy addon and then re-download Dawnguard.
Another shining example of Bethesda's incompetance.....
Same as the fighters stronghold caused the door to the mystic emporium to be permenantly locked in oblivion.....(did they ever fix it? NO. and they won't fix this either.