Many architectural firms do not employ a Registered Communications Distribution Designer (RCDD). You as a network administrator may hold several certifications but this is probably not one of them. A cabling subcontractor most probably has at least one RCDD on staff.

Discovering near the end of construction that a cable tray planned for a specific hallway cannot be installed because of a new HVAC system is blocking the path can be disastrous. This happened to me some time ago, and the result was that some planned Cat 5E cable runs exceeded 100 meters, necessitating the rapid creation of a new IDF at the end of the project.

Ideally, the RCDD will be able to review all building systems as designed by the architect to ensure such problems do not occur. I recommend that part of your design document that you require an RCDD sign off on all architectural plans. Remember, the goal is to reduce changes after construction has started. A not so exact rule of thumb is that the cost for changes increases exponentially as the project progresses to completion.

5. Remember, honesty is the best policy

Finally, while hopefully the above suggestions will reduce the chance for errors on your estimates, mistakes sometimes happen. The worst thing to do is to try to hide the mistake.