Keep the designers informed. It might save the job later. Do not proceed without a detailed architectural program. If you don't have one, create one. Make sure all owner team members with a stake in the outcome are involved in the programming and design.
Nothing is free.
Always make corresponding budget adjustments when changing space or quality requirements or equipment needs. Take soil borings. Take more soil borings. Allow the design team to specify products competitively or negotiate preferred product costs up front. Pay for a detailed cost estimate at the design development phase.