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.