Variations may arise in any of the following situations
(1) when the architect needs or wishes to vary the design or the specification.
(2) when a discrepancy is discovered between any two or more of the contract documents.
(3) when a discrepancy is discovered between any statutory requirement and any of the contract documents.
4) when an error in or omission from the contract bills is discovered.
(5) when the description of a provisional sum for defined work in the contract bills does not provide the information required by General Rule
Few, if any, variations have no effect on the Contract Sum. In order, therefore, to adjust the Contract Sum adequately and satisfactorily within the Conditions of the contract, the quantity surveyor (and the contractor’s surveyor, too, if he wishes) must ascertain the net extra cost or net saving involved. To do that, the affected work will have to be measured, except in those instances where the method of valuation makes it unnecessary, as dis-cussed below. This means that the work originally designed, which will not be required as a consequence of the variation, must be identified, isolated and valued, and that which will be required to replace the omitted work must be measured and valued. Such measurement may be done from drawings or, if drawings are not available, by physically measuring the substituted work on the site after it has been carried out.
This same procedure will be necessary whether the contract is based upon bills of firm quantities or on specification and drawings only. In the case of contracts based upon bills of approximate quantities and those based on a schedule of rates, the whole of the Works will have to be measured as described above. In all cases the valuing will be done in the manner discussed in the section below headed ‘Valuing variations’.