If you plan on playing Dawnguard on a new character and still being able to use Breezehome, don't bother. You cannot purchase the Breezehome alchemy lab with Dawnguard installed. I mean, you can buy it multiple times, and the slick steward will keep taking your 500 gold, but the lab never appears. Dawnguard BETA testers apparently made sure Bethesda was aware of this game ruining glitch, or so I've heard.

Does anyone else have any information on this glitch or when it will be fixed? I know this isn't just effecting me, but everyone. Seriously, go make a new character and try to buy the lab.

No one else has this issue?

It's actually pretty-well documented. Check the other bug report threads.

I've just found the official note in the Bethesda support section. It mentioned nothing about fixing it or current progress on a post-Dawnguard patch.

TEMPORARY WORK-AROUND: If you move Dawnguard over to a flash drive and clear the cache, you can load up your save and buy the Alchemy lab. You'll just lose all of your DLC items and have to redo all the Dawnguard quests. You do keep any non-DLC loot and XP though.