Legal Question in Civil Litigation in California

It depends on the motion but almost always a written opposition is required. In fact in any court I have ever appeared before, if you don't file a written opposition before the hearing, the motion will be granted as unopposed and there will be no hearing, except in the very rare case of a motion that does not require a written response. The deadline for filing the response depends on the type of motion.

Most courts on most motions require a written opposition in proper form, filed by a proper date, with citations to proper legal authority. Timing on motions differ depending on what motion. Many motions require the oppositions filed 9 COURT days (not calendar days) plus more time if you are mail serving the Opposition. See California Code Of Civil Procedure 1005.

Most courts treat the failure to file an opposition as the waiver of the right to argue at the hearing. Further, as pointed out by the previous posts, some courts treat a failure to oppose a motion as a concession that the motion has merit and should be granted. So not filing an opposition is one of the worst things to do, plus it causes a gap in the record that cannot be filled later with what you wanted to say, if you feel the need to appeal.

Generally, oppositions to motions must be filed and served at least 9 court days before the hearing. (Code Civ. Proc., sect. 1005, subd. (b).) As Mr. Selik points out, this time must be increased, depending on how you serve your opposition to the opposing party. Some motions, however, are governed by special rules. Oppositions to motions for summary judgment, for example, must be served 14 days prior the hearing. (Code Civ. Proc., sect. 437c, subd. (b)(2).)