Fire insurance is as we all know a contract of indemnity i.e in case of damage or loss the insured property is restored to the nearly the same. In an ordinary policy of fire insurance the insurer is not actually reinstated the property.Once payment for loss suffered is discharged the insurer's obligations are over. It is not practical for an insurance company to make a valuation of every property which the company insures. The liability rests on insured to state the sum insured.
The method of assessment is to estimate the replacement cost in case of total or partial loss.
Thus according to me the assessment for replacement cost should be found out by making detailed estimate in accordance with the current rate and deducting depreciation considering age, repairs and such other aspects.
Yoy may also work out the salvage value as the subsequent to the insured party's payment,the insurer is entitled to the salvaged property.