A “Solo-k” is a 401k retirement plan for a business with no employees except the owner and, if applicable, the owner’s spouse. The rules are the same as for a traditional 401k except that no annual reporting is required with the IRS until the balance of the plan (not account) reaches $250k. When the plan balance reaches/exceeds this limit, the employer must file an annual 5500-EZ, which is much simpler than the long-form 5500.
Moonlighting income reported on a 1099 is often used to set up a Solo-k. If you own less than 80% of your main employer, you can open the Solo-k as a second retirement account and contribute up to $56k ($62k if age 50+) in 2019. You also are not allowed to open a separate retirement account for yourself alone if you are a partner/shareholder in another business, such as a medical practice.
You must set up your plan by December 31 but you have until the due date of filing your personal tax return, including extensions, to contribute to it.
How much income do I need to set up a SOLO-k?
Small Business Retirement Plans, Part 3: 401k’s
Which is Better: SEP or Solo-401k?