How much can you trust the information on any AS BUILT drawings or documents your clients handed to you for reference ? It could be 99 % or 0.9 % accurate. How about all the things that cannot be seen ? Things hiddened under the slab, ceiling or walls. Do you have a fee structure to deal with all those unknown issues ? Have you explained them to your clients ?
How much time do you need to verify without taking down wall , opening up floor slab or move 10 tons of boxes?
Ok the bottom line is you need to know how to charge and deal with all the unknowns before you start the project . You need to let your clients agreed to all these potential charges before you sign the contract with the clients. Or you should just walk away.