Pricing commercial work is notoriously difficult, and really just comes from practice.
One thing I have done in the past where I was not sure about a building is to say "This is the price for this first time. When I have completed, It might be subject to review"
If you explain to the facilities person that it is difficult to accurately quote until it has been done, they usually understand that and are ok with it. (9 times out of 10 they will accept your revised quote)
Its if you start to fiddle around with prices a few months down the line and dont give a good reason for it you'll get problems, so it pays to be upfront about the issue.
If you're really stuck, you could take some photos of the building and post them for us to see and we'll help as best we can.
-Philip