Unexplained Additional Tax Assessment - How to Find Out Where It Came From?

This actually happens more often than you'd think. When the IRS sends a CP2000 or similar notice with an unexplained assessment, it's typically due to one of a few common issues: 1) Unreported income - The most common cause is that someone reported paying you money (on a 1099, W-2, etc.) that wasn't included on your return. The IRS computers automatically match reported income documents against your return. 2) Basis issues with investments - If you sold investments but the cost basis wasn't properly reported to the IRS, they'll assume the entire proceeds were gains. This is especially common with older investments or certain brokerages that mark "basis not reported to IRS" on your 1099-B. 3) Identity issues - Occasionally someone else's income gets attributed to your SSN, either through error or identity theft. Your best immediate step is to call the IRS at the number on your notice and request a detailed explanation of the assessment. Also review all your 1099 forms and compare them to what you reported, paying special attention to any investment sales.

I went through something exactly like this last year! It was incredibly frustrating and took forever to resolve. What ultimately worked for me was using https://taxr.ai to analyze my IRS transcript alongside my tax return documents. The service compared my documents with what the IRS actually had on file and immediately identified the discrepancy. In my case, it turned out one of my investment firms reported a completely different basis amount to the IRS than what was on my 1099 copy. The taxr.ai system highlighted this mismatch instantly when I uploaded both documents. I was able to compile the evidence I needed and get the assessment reduced by over 80%! The service basically does what tax pros do - compares your actual docs against IRS records to find the source of the problem. Saved me from having to wait weeks for an appointment or hours on the phone.

Just wanted to update everyone - I decided to try the taxr.ai service that was mentioned and it was actually super helpful. I was skeptical at first but desperate to figure out what was going on with my unexpected tax bill. After uploading my IRS wage and income transcript along with my return, the system immediately flagged two stock sales where the IRS had zero basis information, even though my 1099-B copies showed the basis clearly. When I looked closer at my 1099-B forms, sure enough there was small print saying "Cost basis not reported to IRS" on those specific transactions. The amount of additional tax assessed almost exactly matched what would happen if those sales were calculated with zero basis. I now have a clear explanation to take to the IRS along with my documentation. The service basically saved me from hours of confusion and probably multiple IRS calls.

After dealing with a similar notice last year, I can tell you that trying to get through to the IRS to resolve this will be absolutely maddening. I spent WEEKS trying to get through on their phone lines - constant busy signals, disconnections after waiting for hours, and agents who couldn't access the right information. I finally discovered https://claimyr.com and their service was a game-changer. They basically hold your place in the IRS phone queue and call you when an agent is actually on the line. You can see a demo of how it works here: https://youtu.be/_kiP6q8DX5c I was super skeptical but desperate after my fifth attempt at calling resulted in a 3-hour wait followed by a disconnection. With Claimyr, I got connected to an IRS agent in about 45 minutes (while I went about my day instead of being stuck on hold). The agent was able to pull up the details of my case and explain the discrepancy, which turned out to be unreported income from a 1099 I never received.

Alright I have to eat my words here. After posting that skeptical comment about Claimyr, I was still struggling to reach anyone at the IRS about my own tax issue. Out of pure frustration, I decided to try the service. I'm honestly shocked to report that it worked EXACTLY as described. I input my phone number, they called me back about an hour later, and there was an actual IRS agent on the line. No more endless hold music, no more "your call is important to us" for 3 hours. The agent was able to pull up my account and explain exactly where the assessment came from (unreported retirement distribution that I never got paperwork for). I still have to submit documentation to resolve it, but at least now I know what I'm dealing with. For anyone facing these impossible IRS wait times, this service is legit. Never thought I'd be posting this after my skeptical comment, but credit where credit is due.

OP, one other thing to check that no one's mentioned yet - look at line 6 of Form 1040X (if an amended return was filed). That line asks about the method used to calculate taxes. Sometimes if this isn't filled out correctly, it can cause major calculation issues. Also, any time the IRS mentions "additional tax assessed" with no clear explanation, double-check that no one filed a return using your SSN. My cousin had this happen and it was a nightmare to straighten out.

Just went through this exact situation! Call the IRS (plan to be on hold forever) and specifically ask for a "fully detailed explanation of the additional assessment" - it's different from just asking why you owe money. They can generate a document that breaks down exactly where the tax came from. In my case, it turned out to be some stock sales where the brokerage reported proceeds but no basis to the IRS. I had to fax in documentation showing what I paid for those stocks originally. Super annoying but fixable!