You don't necessarily need an attorney to respond to your CP2000 notice but it wouldn't hurt from the aspect that an experienced has been there before. I suggest that you always respond to such notice via U.S. Certified Mail, Return Receipt Requested in order to ensure that they receive your responses. Lastly, the IRS cannot simply assess additional tax from a CP2000. They have to send another notice called a Notice of Deficiency before they assess the tax (assuming that you don't agree to the assessment.) When you get the Notice of Deficiency you have the right to file a Tax Court petition before the tax is assessed. It's far easier to "win" at the CP2000 level so you want to give it your best shot.