I respectfully disagree with my colleague. I assume this is a regular civil matter filed in a Superior Court of CA and there are no court orders to the contrary. If so, you may serve discovery responses in any manner permitted by law, including regular U.S. mail. Service by personal delivery merely requires that you respond within 30 days of personal service (by any manner permitted) as opposed to being given a longer period of time to respond because service was by another method. One caveat: a party cannot serve the documents or sign a proof of service.
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I agree with Mr. Daymude. You may serve by any legally valid method, regardless of how you were served with discovery. You have 30 days to respond, unless you call and get an agreement to extend the date, if you need more time. However, they do not have to grant the request. If you mail your discovery requests, the other side will have 35 days to respond.
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Regardless of the method you use to respond to discovery (mail, personal service, etc.), always make sure that you can prove when you served it. With mail, this means spending an extra $4 and having it mailed certified mail + return receipt requested. Then, you'll get the little green card back that will prove that you mailed it and when it was received.
Also remember that if you have any objections, you have to state them within 30 days unless an extension is agreed to, else your objections are waived. If you get an extension, make sure that it is in writing, and that it is an extension as to both answers and objections.