Did you do the whole skooma quest?
Spoiler
Give the lady a potion, than have her tell you who the dealer is
Talk to the Jarl, than clear out the warehouse.
Pick up the shipment ready paper and read it
Talk to the Jarl
Clear out the bandit den
Talk to the Jarl
Than you have to do like 2-3 favor quests
Spoiler
Easiest are:
Do the Ore Sample errand for the old lady in the Alchemy Shop, than can run messages for Stlygia (sp?) and clear out the mine
Talk to the Jarl's son and pick up the blade from the smith (will be to the side of the keep practicing with the guards)
Talk to the Redguard that was arguing with Sapphire when you entered (may be in the stables), than talk to Sapphire
After that, the Jarl should have a "is there anything else I can do" chat dialogue. She tells you to buy the house.
Save here, buy the house, upgrde it, than talk to the Jarl and become a Thane. Go check out the house to make sure everything upgrades like it should.