I live in California and own a house. I paid 13,500 in Mortgage interest. I know I can only deduct 10,000 federally. Can I deduct the difference on my state return?

You are confused ... the mortgage interest is NOT limited by the SALT act ... the total of the  State income taxes + RE taxes + personal property taxes  cannot exceed $10K.

Thank you.  my question is can i deduct the difference on my state return?

California does not allow a deduction of state and local income taxes on your state return.