California Rule of Court 3.110(d) entitled "Time for service of complaint, cross-complaint, and response" says the following:
(d) Timing of responsive pleadings The parties may stipulate without leave of court to one 15-day extension beyond the 30-day time period prescribed for the response after service of the initial complaint.
This allows you to contact the opposing party and ask for an additional 15 days to respond. If the opposing party will not agree, then you can file a motion under part (e) below.
(e) Modification of timing; application for order extending time The court, on its own motion or on the application of a party, may extend or otherwise modify the times provided in (b)-(d). An application for a court order extending the time to serve a pleading must be filed before the time for service has elapsed. The application must be accompanied by a declaration showing why service has not been completed, documenting the efforts that have been made to complete service, and specifying the date by which service is proposed to be completed.
(f) Failure to serve If a party fails to serve and file pleadings as required under this rule, and has not obtained an order extending time to serve its pleadings, the court may issue an order to show cause why sanctions shall not be imposed.
If you fail to respond, the opposing party will file a motion for default as described in parts (g) and (h).
(g) Request for entry of default If a responsive pleading is not served within the time limits specified in this rule and no extension of time has been granted, the plaintiff must file a request for entry of default within 10 days after the time for service has elapsed. The court may issue an order to show cause why sanctions should not be imposed if the plaintiff fails to timely file the request for the entry of default.
(h) Default judgment When a default is entered, the party who requested the entry of default must obtain a default judgment against the defaulting party within 45 days after the default was entered, unless the court has granted an extension of time. The court may issue an order to show cause why sanctions should not be imposed if that party fails to obtain entry of judgment against a defaulting party or to request an extension of time to apply for a default judgment within that time.
My suggestion is to contact the opposing party and tell them you intend to file a response tomorrow. If they give you trouble, go ahead and file a motion with the court for time to answer and attach your proposed answer to the motion. The court should allow your answer to stand. Courts generally do not want cases to go off on a default.
I suspect that the opposing party, if represented by an attorney, will stipulate to the late filing. Feel free to ask any follow-ups.
You need to act fast because the opposing party only has 10 days to request a default. Don't waste any time.