Can I use rental income to fund solo 401k? SEP-IRA?

[John Doyle]

I love maxing out our tax-advantaged accounts (my 401k at work and Roth IRAs for my wife and myself).  

Usually we have a few thousand dollars each year to invest after we max out these accounts (and it usually just goes into a taxable brokerage account where we use it to buy Berkshire Hathaway or Markel or some other stock that does not pay a dividend).

That extra money was actually how we ended up buying this duplex - thought it might be a good way to diversify our investments.

I was wondering if I could use net income from our duplex (assuming we have net income, and by net income I mean what we report to the IRS) to fund a solo 401k or some other kind of tax-advantaged plan?

I understand that these plans are for self-employed individuals or those running a side business of some kind.  Does running a rental property qualify as "self-employment"?

Note: I am not interested at all in using a retirement plan to BUY more properties...just to put money into it a tax-advantaged way and then using that money to buy stocks/ETFs.

Thanks!