The architect should always reserve the right to withhold part or all of the certificate of payment when defective work is not remedied, or when any claims are filed, or there is reasonable evidence that claims will be filed, or when the contractor fails to make payments to subcontractors, or to dealers for materials, or when there is a reasonable doubt that the contract can be completed for the balance unpaid, or when any damage involving liabilities has been done by one contractor to another. The architect should also hold back the final payment, if there are any liens existing against the building, until they are removed. In order to avoid many of the trivial and annoying expenses which occur in a building operation, the contractor should be required to pay for all permits and licenses (but not permanent easements) which are necessary according to local laws. The contractor should also be made to pay all royalties on patents, if there are any, and all license fees. But, probably, the most difficult part of the building operation to finance are the extras. When something is found to have been omitted from the plans and specifications, and the contractor did not cover it in his bid, or when the owner changes his mind and requires an alteration, then this extra work must be paid for at a high rate, for nearly all contractors look upon such extras as good pickings. In fact, there are some contractors who deliberately go over the plans and specifications to note what extras may be needed, and then counting upon their profits from these extras, they put in a low bid, so that they can beat their competitors, secure the job, and then proceed to make up their losses with bills which they put in for the extras. Likewise, a contractor who is honest, if he finds himself losing money on any building operation, will try to ease his losses and gain profit with the extras. There must, therefore, be some basis upon which estimates for these extras will be determined. The values for these extras or changes in the work may be determined by a submitted estimate and acceptance in a lump sum, by a unit price named in the contract or subsequently agreed upon, or by the cost and percentage, or by the fixed-fee method. If the contractor claims that any instructions, by drawings or otherwise, involve extra cost under his contract, he should be required to give the architect written notice of it before proceeding to do the work, within two weeks after receiving such instructions.