An inventory that burns is nothing.

Keeping the inventory in the building it documents. A folder in a drawer goes with the house; photos held only on a phone that was inside go with it too. A copy has to exist somewhere the fire or the burglar can't reach — cloud storage, emailed to yourself, or a drive at work or with a relative. Sort that before recording anything, because otherwise the work gets done and then stored in the one place that guarantees it's useless.

By category rather than by guessing a round number, and priced at what replacement costs now rather than what you paid. The shortfall almost never comes from the obvious items — it's the aggregate: an entire wardrobe of clothing, the full contents of a kitchen, every book and toy and towel, tools, garden equipment and accumulated small electronics. On typical figures those categories are close to half the real total.

No, and this is the part that catches people. Many policies apply a proportional reduction where the sum insured is too low, which means a claim for a single stolen item can be cut by the same proportion as the overall shortfall — not just a claim after a fire. The exact treatment varies by policy, so it's worth asking the insurer in plain terms what happens if the sum turns out to be too low.

Because most contents policies cap what they'll pay for any single item, and anything above that cap has to be specified to be fully covered. The cap varies by insurer, so it needs reading rather than assuming. Worth checking the valuations too: insurers frequently require a recent one for high-value items, and precious metal and stone prices move enough that an old valuation can leave something underinsured even though it's correctly listed.

Film it. A narrated video takes roughly twenty minutes for a whole house against several hours of typing, and it captures things nobody would think to list. Open wardrobes, drawers and cupboards on camera, say the make and model aloud where you can see it, and cover the loft, garage, shed and garden. Then build the written list from the footage sitting down, and add serial numbers for electronics, tools and bikes.