If you lend people money to buy a house, How much interest can you legally charge?

well, there are a couple of issues at play. 1st, the short answer is whatever agreed upon by the two parties. i would highly recommend against this lending, and if you decide it is wise, i would get an attorney involved. i assume you are not an attorney, b/c you would already know the answer.

one caveat is the usery laws. while i am not a lawyer, you can't take advantage of people. for example, charging 30% apm etc. but, you have probably already gone through all the risk analysis of what you will take on, know all the legal hoops for foreclosure, calculated the cost of an attorney and your NPV of your "investment." my guess is after calculating all this, you're better off in an index fund. REIT's if you must invest in real estate. and i hope this isn't to a family member or friend...

why you would want to start a company is beyond me. i see no advantage and many disadvantages if this is a one time thing.

If it is more than one person and you charge interest you will have to pay taxes on the profit you make so it is better to legally open a business !

I do not think you can charge more than what the legal lending interest limits for your state are.Bank could tell you.

There is some point where your state's usury laws would kick in, but that would be pretty high relative to today's interest rates.