my land we are about to build on here in WA and my farm land back in ND are both covered by my homeowners policy.
Once we start to build we then have to buy a rider policy for that. My agent explained it like the auto policy and towing a trailer. The trailer is covered under the auto policy and the land is covered under homeowners.
Kubota B2410 Bi-Speed Turn, R4's, Top 'N' Tilt, LA 352 Loader w/ Toothbar
Assuming they have extended the liability from your homeowners to the additional locations, like adding a trailer to an auto policy, it would seem they are correct.