Source: CA R&TC Sec 19172, 19172.5 and like IRC.

Significant penalties apply to pass-through entities which miss their extended filing deadline of September 15, 2017.  After that point, the extension is not valid.  The penalty for a partnership with two partners filing on November 15, 2017 would be:

Federal: 8 months * $195/partner* 2 partners = $3,120

California: 8 months * $18 * 2 partners = $288

Total late filing penalties based on number of partners = $3,408.

DON’T MISS THE EXTENDED DUE DATE OF SEPTEMBER 15! (Be sure we have your partnership or corporate information no later than September 1, 2017 for the 2016 tax year).