I've played Skyrim five times now, and this is my sixth playthrough. This is the first time I've joined the Stormcloaks instead of the Empire. I heard that it could affect who sells you land, so I thought that was worth mentioning. Anyway, I've become a Thane and talked to the Jarl's husband- whom i thought I was supposed to talk to- but the option doesn't appear for me to buy land. Why? Can I do this?
Indeed, depending on which side you joined the war, the place you'll buy will change, but not your ability to do so, as this article on Elderscrolls wikia points out.
I assume you're trying to buy the land to build "Windstad Manor". For future reference, I'll describe everything you need to do to get it.
Step 1 : complete the side quest
In order to be able to purchase it, you need to complete the side quest called "Laid to Rest", described in this wikia article.
Step 2 : talk to the steward
Then, you have to talk to the steward, between working hours (8am to 8pm). A player on Gamefaqs said "he would go at noon, just to be safe", in his answer to that question, here.
You will finally be able to begin your construction.
The information in this answer is INCORRECT. That is how the game is supposed to work but it does not which is why the OP posted here. There is apparently a bug if you side with Stormcloaks in the civil war then the new steward is Pactur who will not sell you land. I am not aware of any workaround.