Though it can take a while to build up enough cash to put a 20% down payment on an investment property (the typical lender minimum), they can snowball fairly quickly. The key here is to correctly project income and expenses in order to calculate cash flow (the free cash you can put in your pocket after all associated property expenses have been paid). However you have to be sure to include the cost of a property manager in your calculations unless you want to manage the property yourself. Even with a property manager, you may be required to make large repair decisions every now and then – so while this is not a 100% passive activity, you are not directly trading your time for money like traditional employment.

I read about early withdrawal penalties on IRAs/401Ks very often. Almost always with a statement of “locked up” or “can’t touch” until 59.5. I’m sure you and well informed readers as well know about SEPPs in regard to IRAs/401Ks. For those that don’t SEPPs aren’t perfect but they are a way to tap retirement funds penalty free and I will be using in the future as I have over half of my equity investments within retirement accounts. South of a mil, North of a half. Let me add that I think your blog is outstanding.
The biggest surprise is real estate being second to last on my Passive Income Ranking List because I’ve written that real estate is my favorite investment class to build wealth. Physical real estate doesn’t stack up well against the other passive income sources due to the lack of liquidity and constant maintenance of tenants and property. The returns can be huge due to rising rental income AND principal over time, much like dividend investing. If you are a “proactive passive income earner” like myself, then real estate is great.
Blogging – I guess you could say I’m a professional personal finance blogger since I own two sites and I’m making decent money every month. The income started off slow but has been consistently increasing. It’s not as much as I make with my day job but my best blogging month was equal to about one paycheck at my old day job.  While I had to learn how to set up and use WordPress myself, you can learn how to blog and make money online at StartABlog123.com.
Capital growth in your portfolio can offset the eroding effect of inflation.  But any capital gains that exceed the overall inflationary effect can be drawn off to augment your portfolio income.  Of course, you must first generate those gains by making thoughtful investment selections.  While allowing strongly growing assets to keep doing that, it is prudent not to allow them to become a dangerously large part of your portfolio lest they go off the boil.  So if you selectively trim profitable positions along the way, you can boost your income.
In January 2018, I missed my chance of raising the rent on my new incoming tenants because it didn't come to mind until very late in the interview process. I didn't write about my previous tenant's sudden decision to move out in December 2017 after 1.5 years, because they provided a relatively seamless transition by introducing their longtime friends to replace them. I didn't miss a month of rent and didn't have to do any marketing, so I felt I'd just keep the rent the same.
Or, there is another theory for your primary salary – generate enough to have a little excess cash flow, but do it at a place that you can work stress free and have time to dabble in other projects.  A good friend of mine has this setup – he works 10-5 and makes $50,000 a year.  This allows him to easily cover all of his expenses, but the shorter hours and flexibility in his job allows him to pursue his secondary income generating ideas!
For a person working for a wage, their source of income is their job, or their labor (aka their time that they sell to someone). But you can own a house, and rent it out, and when your renters pay you renter every month, the source of that income check is your rental property, or your house. It would just be called rental income. If you have money in the bank in an account that pays interest, those interest payments are another source of income. We can say the source is interest payments, or we can identify the asset that is generating rent, and say your money in the bank that is generating “rent” (aka interest) is your source of income.
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