kzam got real close to stating my experience. I had an old U-Haul trailer that I modified to go off road some years ago. It was licensed, but I was told by my agent (S.F.) that when the trailer was parked on my property, homeowners policy covered it. When it was on the road, it was covered under the policy of the tow vehicle. I never did get any trailer insurance.
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As an aside, why in the world would you tell an insurance company that your trailer was "home built"? They don't need to know that! If you have it registered/licensed, that's all they will need to know. Again, in Cali, little trailers don't (didn't?) need insurance... unless something has changed (possible).