BIM for existing buildings

One of our current projects is the rehab of an existing building. We are going to utilize BIM on this project.

The first step is modeling the existing structure. There are several issues with that. The obvious one is guaranteeing 100% accuracy. This is especially important because the owner intends to panelize the interior partitions off site. The potential solution for this will be as follows:

• Model structure to remain from original as-builts
• Field verify the overall geometry of the model; update as needed
  
• Laser survey the existing conditions; update dimensions/locations of columns in model based on survey; update as needed.
  

This, of course, causes several other issues. The most critical is finding a surveying company that can do laser surveying that we need. Other issues involve creating all of the columns for the building.

So far, 20 custom columns have been created for the project and the first floor isn't even finished yet.