One of the things that needs to be taken into consideration here is when you set your business plan, as anyone looking to start a legit business should, your business is/should pay rent for the space reguardless of where it is located. Granted, you may not charge your business the same rental rate for a room in your house as a location outside your house, but there should be a line on your balance sheet with a number dedicated to studio/office rental. This does 2 things, one it pays you for the space your business is taking up inside your house, and two if you grow to a point where you need to move out of you house there is already a portion of your income dedicated to rental so you don't have to make dramatic changes to your pricing to compensate for new space.
As KMH said too, having a studio greatly increases overhead expenses so although you might have a higher income on paper, your actuall net income will most likely be lower. This is all given the fact that you are structuring a business with a plan, taking into consideration all the expenses that go along with it. If you are just doing under the table type of work, the added overhead of having a studio may never actually be felt because you probably aren't following the numbers that closely, just looking at what each job brings in on it's own.