Although a valuer is not expected to know detailed rules of law, he/she does profess to know the general rules of law under which a valuation is made:

We agree with my learned Brother, that the defendants could not be expected to supply minute and accurate knowledge of law; but we think that, under the circumstances, they might properly be required to know the general rules applicable to the valuation of ecclesiastical property, and the broad distinction which exists between the cases of an incoming and outgoing tenant, and an incoming and outgoing incumbent. According to the evidence, it is plain that the defendants acted in the valuation as if it were the case of outgoing and incoming tenant merely, and that they knew of no other rule. Their ignorance in this respect was a breach of their previous engagement, to have found a verdict for the defendants - Jervis CJ in Wise v Metcalfe (1855) 15 B&C 168, 189.