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During construction, the contractor A builder or other trade who carries out the construction work under a construction contract. A contractor can be a head contractor or a subcontractor. View full glossary may propose substituting materials and products specified in the contract documents Contract documents include: a particular written contract, including the agreement between the owner and the contractor, any special conditions, the drawings and specifications, and any other relevant documents. View full glossary . Caution must be taken when reviewing and accepting substitute products as this activity represents a significant risk to architects. Considerable time, research and design effort goes into documenting a design and selecting materials and products to ensure their suitability, conformity to building codes and standards, environmental and cost considerations and fitness for purpose. Often, through the design process, material and product selections have been reviewed by sub-consultants, fire engineers, building surveyors and regulatory authorities to confirm their suitability.