the approach i take is similar. what i do is setup a seperate file for all my as-built work. in this way i can always have the latest survey information for the existing structure that is separate from the new, i can model the existing cleanly, and control any demolition aspects quite effeciently. i also use stationary files for this work where i have modified vw's classes to clearly indicate whet is existing. for example, Door-Exst, Window-Exst, Wall-Exst. i like to be able to control the appearance of the existing stuff this way. i also modify the layer names such as Mod-Floor-1E. this file is also good as a stand alone for doing area calculations, facility management tasks, and issue for any consultants.
then i creat a new file that will be where i draw all the proposed stuff. i simply wg ref the required layers from the referenced survey file into the propsed target file. i have found it to be the simplest way to organize existing and new drawings. i hope this helps...