If this is a tax lien, you may have a longer wait than 12 months. Once you buy a property, the IRS has claim to seize the home as collateral to their proprietary claim. FHA (the most lenient of the bunch) requires the IRS to agree to subordinate their lien to the mortgage.
Does the tax lien show on your credit report as a lien? If so, the bad news: I've been doing this a long time and have yet to see the IRS subordinate a Notice of Federal Tax Lien to a purchase money mortgage. The only
successful subordinations I
have seen have been for refinances to obtain a lower rate with a strong payment history of more than one year (and even then, we had to get the borrower's State Representative in Congress to help). I'm sure others have seen these agreements, but I would be curious if they have seen them in recent years. (emphasis on what I have seen, we hear of others being successful, but when asked "have you personally seen this" I have yet to see anyone say yes - and then, I would want to know the investor used).
However, if you are on the IRS payment plan, with no legal filing (not on your credit report), you may be able to get away with a 12 month payment history. Be sure you answer this question correctly: Are you presently delinquent or in default on any Federal debt, or any other loan, mortgage, financial obligation, bond, or loan guarantee?
Keep asking around......push to get it in writing once you hear someone say they can do it........get the loan pre-approved with all of your documentation reviewed. Once you have it in writing, you can take it to the bank. And if it's subject to IRS subordination, you know you have your work cut out for you. (IRS publication 785 - http://www.irs.gov/pub/irs-pdf/p785.pdf