Rebuild cost and market value are different numbers.

Market value is set by what buyers will pay for the land and the house together, in a particular location, at a particular time. Replacement cost is set by builders’ rates, materials, labour availability and the building code. A well-located but modest house can be worth far more than it costs to rebuild; an architecturally complex or heritage house in a soft market can cost considerably more to rebuild than it would fetch.

Insuring on the wrong basis produces one of two bad outcomes. Under-insure and a claim is reduced — many policies apply an averaging or co-insurance provision, so the shortfall is borne proportionally by you even on a partial loss. Over-insure and you pay premiums for years on cover you can never claim, because the insurer will only ever pay the cost of reinstatement.

Neither outcome is visible until something goes wrong, which is precisely why the number is so often left as a guess with a decimal point in it.

Demolition and site clearance. After a total loss the site has to be made safe and cleared before anything can be built. On a sloping or access-constrained site this is a material figure on its own.

Professional and statutory fees. Architect or draftsperson, engineer, building certifier, council approvals, and surveys. These are a real percentage of the rebuild, not a rounding error.

Compliance with the current code. A house built in 1975 cannot be rebuilt to 1975 standards. Energy efficiency, bushfire attack level, flood immunity, wind rating, glazing, insulation, plumbing and electrical all have to meet today’s requirements — which can substantially exceed the cost of replicating what was there.

Escalation and time. A rebuild takes many months, sometimes far longer after a regional catastrophe when local trades are stretched. Costs move over that period, and the assessment has to account for it. Temporary accommodation and loss of rent are typically separate policy items, but the rebuild timeframe drives them too.

Any time you have renovated, extended, or changed the property materially — the sum insured almost never keeps pace with a renovation, and a decade of small changes adds up.

When your policy has been rolling over on an indexed figure for several years. Indexation is a blunt instrument and does not know what happened to your house or to builders’ rates in your area.

When the property is unusual: heritage fabric, non-standard construction, a difficult site, significant landscaping or outbuildings, or a dwelling that would need specialist trades to reproduce. Generic calculators handle none of these.

And before a claim, not during one. An assessment prepared while the house still stands is evidence; one prepared afterwards is reconstruction from memory and photographs.

What a replacement cost assessment costs

Is replacement cost the same as market value?

No, and confusing them is the most common cause of underinsurance. Replacement cost is what it would cost to rebuild the improvements today, including demolition, professional fees, code compliance and escalation over the rebuild period. It excludes the land entirely. Market value is what the whole property — land and house — would sell for. The two move independently and are frequently very different figures.

Can I just use my insurer’s online calculator?

For a standard project home in a metropolitan suburb, a calculator may get reasonably close. For anything else it is a rough estimate built from averages: it has not seen your house, does not know its construction, condition, access, site constraints or finishes, and cannot assess what current code compliance would cost on a rebuild. Where the property is unusual or the sum insured is significant, an assessment based on an actual inspection is a different quality of evidence.

Does the assessment include the land?

No. Land is not destroyed by fire or storm, so it is not insured and is not part of a replacement cost assessment. This is one reason the figure is often lower than owners expect — in many locations the land is the larger part of the property’s market value.

How often should the figure be reviewed?

Whenever the property changes materially — a renovation, extension or significant outbuilding — and otherwise periodically, because builders’ rates and code requirements both move. A sum insured that has only been indexed for many years is worth testing, particularly after a period of sharp construction cost movement or a regional event that has stretched local trades.