Looking for guidance on incurred penalties

Hi all – this is my first year dealing with 1099 income and trying to understand an underpayment penalty I just got.

Timeline:

• W-2 income ended June 2025 (single, claimed 0) 

• Switched to 1099 income starting 9/20/25 (no withholding) and did not receive any client payments until after September quarterly deadline 

• I made no estimated payments made during Q3 or earlier in the year

In early January, I gave my accountant all my income info (W-2, 1099, dividends, stock sales, expenses). Based on that, I made a single estimated tax payment by 1/15/26, which he said were on the high side. These are all of the same figures used to complete my April return.

My return is now complete and I owe ~$600 in penalties.

Any guidance on:

• Is this likely just an underpayment penalty from missing quarterly estimates?

• Would using the annualized income method have reduced or eliminated this?

• Should I have made a Q4 payment by 1/15 and earlier catch-up payments?

• Anything else I should be asking my accountant about? This is my first time working with him and/or any accountant.

Appreciate any insight—just trying to understand what happened so I can handle this better next year.