This is from AI Appraisal of Real Estate, 11th Edition, page 89:
In the valuation model, land value estimate is a separate step. The most reliable
way to estimate land value is by sales comparison. When few sales are available, however,
or when the value indications produced through sales comparison need additional
support, other procedures may be applied. The procedures used to obtain land value
indications are: sales comparison, allocation, extraction, subdivision development, land
residual, and ground rent capitalization. These techniques are described as follows:
1. Sales comparison. Sales of similar, vacant parcels are analyzed, compared, and
adjusted to provide a value indication for the land being appraised.
2. Allocation. Either sales of improved properties are analyzed and the prices paid
are allocated between the land and the improvements, or comparable sites under
development are analyzed and the costs of the finished properties are allocated
between the land and improvements. Allocation can be used in two ways: to
establish a typical ratio of land value to total value, which may be applicable to the
property being appraised, or to isolate the value contribution of either the land or
the building from the sale for use in comparison analysis.
3. Extraction. Land value is estimated by subtracting the estimated value of the
depreciated improvements from the known sale price of the property. This
procedure is frequently used when the value of the improvements is relatively low
or easily estimated.