Your business must have some active business activity in order to adopt the Solo 401k.

Once you’ve started running your business, you should have an intent to profit from your business and receive earned income. Earned income is defined as money derived from paid work.

To get some more clarification between earned income and passive activities, check out the passive activities examples worksheet, according to the IRS (download here).

For more information as it pertains to your specific situation, check with your CPA.
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