A house can be insured for more than it would sell for, or for considerably less, without either figure being wrong. The two numbers measure different things.
Market value includes land, insurance does not
A sale price covers the land, the location and the structure together. A buyer in a desirable area is paying largely for the ground the house sits on.
Land does not burn. Insurers exclude it because it survives almost every peril a homeowner policy covers.
A property whose worth is mostly locational will therefore insure for far less than it sells for, and that is the intended result.
The reverse happens too. In markets where land is cheap, a modest sale price can sit well below what rebuilding the same house would cost today.
Replacement cost is a construction estimate
Replacement cost is what it would take to rebuild the structure to its current condition using materials of like kind and quality at today's prices.
It is derived from square footage, construction type, finishes and local labour costs rather than from comparable sales in the neighbourhood.
Where construction is expensive and property is cheap, that figure can exceed the market price of the finished house by a wide margin.
Building codes add cost that valuations miss
A house built decades ago may not comply with current requirements for wiring, insulation, roofing or structural bracing.
Rebuilding after a loss must meet the code in force at the time of the rebuild, which can be substantially more expensive than replicating what stood there.
Ordinance or law coverage exists as a separate endorsement precisely because standard replacement cost does not include that increase.
Underinsurance triggers a proportional penalty
Policies commonly require the dwelling to be insured to a stated share of replacement cost, and falling below it reduces payment on partial losses proportionally.
The penalty applies to ordinary claims, not only to a total loss, which is where owners tend to encounter it unexpectedly.
Extended and guaranteed replacement cost endorsements exist to absorb estimation error, within limits stated in the contract.
Neither endorsement removes the need to keep the underlying amount current, because both are calculated as a margin above the figure printed on the policy.
Both figures move, and not together
Construction costs respond to material prices, labour availability and demand surges after regional catastrophes, when many houses need rebuilding at once.
Market values respond to interest rates, local supply and neighbourhood demand, none of which change what a builder charges for framing and roofing.
A policy amount set at purchase and left alone drifts away from both, which is why insurers apply inflation adjustments at each renewal.