As above...
I've read so many conflicting things I dunno what the hell to believe!

I need help too. im not even geting the option to become the thane, but i can still help out everyone. do you NEED to be a thane to get the offer for the plot of land?

oh and i havent done the DB quest at all or the civil war, and just started the MQ

If you killed Jarl Siddgeir's housecarl from the dark brotherhood quest, then you can be bugged, even if you killed him in secret. Same thing happened to me. I even checked to see that I had the Blade of Falkreath yet when I tested and committed a crime in town, my Thaneship was gone. Something bugs out and sets your reputation level with him to bad.

In order for me to fix this, I had to get the Jarl and Steward replaced. I did this by siding with the Stormcloaks and it fixed the problem for me. I talked with the new steward (old maid lady) and the dialogue that spawned was if there were any houses available for sale. She will say no but there is a plot of land for sale. I have built my home in Falkreath as my second.

For others that didn't do the dark brotherhood, then there are other critera's to be met. You have to make sure to do the Jarl's fetch quest (if it's Siddgeir) and do the bandit/target killing quest. If you don't have the option to do the fetch quest, check your misc quests and make sure you are not on another fetch quest for someone else. It will not spawn until you have cleared it out.

No I have gotten the falkreath home without being thane after i do the jarls first radiant quest....

thats the thing though the jarl is not giving me the quests to begin with.

I have the Falkreath home with my lvl 12 character. I hadn't started the DB Quest yet nor the Civil War one or the MQ. All I did was go to Dragon Bridge and hung around there for a little and a courier came to me and told to go talk to Falkreath thane etc. I'm still not Thane of Falkreath and I have a completed Manor..

once you kill the bandits for him, he should say something like "I like you so i'll allow you to buy land in my hold" that may not be the exact words but it's along them lines. then you have to speak to his steward the high elf (cant remember her name) and the option to buy the land should be in her dialodge

You can't be on any other "rare gifts" fetch quests as it will lock out the fetch quest for the Jarl and his black briar mead.

You can see the list here http://www.uesp.net/wiki/Skyrim:Rare_Gifts

thanks for the link. right now im trying to get the book the mirror, i dident now that that could stop me from getting another quest im going to finnish it whan i get on next and see if that help. do you know why it locks out the other quests?

Unfortunately I don't have the answer. Just one of those Beth "huh" decisions