Other land locked parcel sales can be considered in order to determine the appropriate adjustment necessary to the non-land locked parcels that are comparable to the subject except for their having access which the subject does not have.
PE:
I was referring to a bilateral monopoly scenario, which would be a landlocked scenario, but doesn't apply to all landlocked scenarios.
In the bilateral monopoly scenario, the valuation is really simple: The maximum value is the contributory value to the potential buyer's site: a rational buyer, albeit he or she is the only buyer, will not pay more for the parcel than what it contributes to the existing parcel (or assemblage's) value.
The minimum value is zero (theoretically, the seller might have to "pay" the buyer something to take the parcel off their hands... but let's not get into negative value! :new_smile-l

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So I'm not contradicting your general statement regarding like-for-like (landlocked subjects are best compared to landlocked comparables).
I am saying, that by definition, a bilateral monopoly is unique unto itself (one buyer, one seller, one unique property); in that case, another sale, landlocked and even bilateral, wouldn't provide an indication of market value for the subject.
(at least, that's what I recall from the course when we discussed the bilateral monopoly scenario! :new_smile-l

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