@Steven WilsonGood point.I don't think it is viable as your sole investing strategy but I think it could be a great way to supplement some extra cashflow if you have a garage you don't use or a big yard to park an RV. 

When you look around the world the countries that have all these freebies have high taxes so the productive in society are just supplementing those that are not. 

House-hacking can be a great strategy to supplement your housing payment and I hope it works out for you.

At first, I wanted to passively supplement my income with rental properties, but after thinking about it I want to actively invest and build a portfolio.

Begin increasing monthly income to supplement day job.

Ball valves are cheaperBooster-seems to be just for supplementing, not clear if it can heat by itself?   

Schnapf  I agree 100% whenever there is an environmental issue with a property and the state will fund or supplement the remediation, its worth going through a little extra red tape!

If you have some money to work with, even though the market is competitive you can find something that is a great investment and can supplement your income, allow you to live "Rent free", and sooner or later cash flow.

Purchase at least as much as your net sale (contract price minus closing costs and commissions)You can use your own cash to supplement. 

I've gotten some leads and closings which has supplemented by existing book of business but definitely by no means mastered it.