How can I disentangle the cost of land from the cost of the house structure (for homeowner's ins.?).

The answer in from "sunnyview" is almost completely immaterial. A homeowner's policy does not inusre the land, as the land itself is uuninsurable (except for the a small amount of coverage for trees & shrubs). The policy will only insure the dwelling and separate structures.
The value of the land matters nothing.

Your insurance agent will calculate your replacement cost of your home and will insure based on that amount. Most policies automatically include an extra 20-25% above that amount in case the estimate is wrong. These two amounts togather will be the maximum paid in the event of a claim on 95% of home policies.

However, the best policies, often only available on homes with high replacement values, will insure for full replacement cost, with claim payouts not limited by the replacement cost calculation. Insurers typically send then own inspectors to do a detailed inspection and replacement cost estimate so make sure they have adequate premium for the exposure. 

The best shortcut for that lies either with your insurance agent who has a breakdown of land vs structures or you can also look at your assessment information for land value vs structure values. Chevy Chase is a very nice area and I understand your desire to cut your insurance bill, but please make sure that you don't underinsure. Rebuilding costs can be higher than anticipated especially in areas where permit fees and permits are costly. Insurance will cover basic rebuilding cost,but often does not cover things like hardwood floors, updating electric in older houses etc. Some policies do and some don't so check with your agent to be sure before you decide to reduce coverage.

I would start with the land value on your insurance policy and compare it to the number for land on your assessors rolls. That should give you a place to start. If you need more information, call a local agent to see if there are any lots or teardowns for sale in your area. That would give you a base value to go from for land. In an established area like CChase that might be hard to find, but it's worth a try.