I have rental income in my primary residence (I rent a room to a non-family person). If I chose to declare this income, what is my tax liability? California

[AmeliesUncle]

"If" you "choose" to declare the income???  It's not optional.

Without knowing how much your profit is, what you other income is, and a bunch of other things, nobody could tell you.

Enter your ALL of information into the program, and it will tell you the results.

[Carl]

Curious as to why you believe you have a choice here, to report the income or not. That "might" be a choice if it was a family member. Then it might qualify as a cost sharing arraignment. But not when renting to a non-related person. You're required to report the income, like it or not.

If this is a long term arraingment, then it's reported on SCH E. For a short term arraignment (such as AirB&B or VRBO) it may or may not qualify as a SCH C business. But either way, the income is reported on both federal and state taxes if your state taxes personal income.

[Hal_Al]

If this is merely a cost sharing arrangement where the amount paid is below fair market rental, there would be no reportable income to you. If the “rent” amount is fair market value, or more, there is still some question as to whether you even have to report it, as it almost always comes out zero. Most people take the attitude that it is not income; it's just room mates sharing expenses and ignore it. Family, as opposed to unrelated roommates, makes that position stronger.

Here’s what you may be required to do:

Report the income (enter at Rents & Royalties/Income & expenses from Rental Properties); and then deduct the expenses on schedule E. If the room mate has full run of the house, and there's just the 2 of you, then half your expenses are deductible (mortgage interest, property taxes, insurance, utilities, repairs, and depreciation [if needed}). Your net income will usually be less than zero.

What you are NOT allowed to do, because it is your own home (you have "personal use") is claim a loss from this activity, to offset other income. Because of the "personal use rule", your deductions are limited to your income. Net effect ZERO.