PA28 pilot is very confused by new ramp construction and diagram provided by FBO. Reporter is unable to locate taxiway C and ends up on D headed Northeast.

Narrative

A new ramp and run-up area had opened within the prior 2 days at FBO X. Details are not in current AFD for modified taxi diagram. On checking out rental aircraft; I asked for any new taxi instructions/procedures. I was provided with a photocopy of an aerial photograph indicating ramp traffic flow and taxiway entrance to ramp. After aircraft ground check and run-up; I received clearance to taxi to Runway 27L at Taxiway C. From the ramp/run-up position; there could be seen a direction sign indicating 'Charlie' to left and right. I proceed to the right to follow the currently unnamed taxiway; across the threshold of inactive (on the day) Runway 5 to intercept Taxiway C; to then proceed to Runway 27L. At this point; Ground clearly instructed me to turn left onto 'Charlie' as I was now on 'Victor' and needing to proceed on 'Delta' to the 'Charlie' intersection. I proceed to Runway 27L at Taxiway C; requesting confirmation prior to crossing 'Alpha' that this was my destination; as there was another aircraft taxiing under progressive instructions from Ramp. This was confirmed by Ground. 1) Problem: Hand marked ramp/run-up diagram was misleading. Taxi diagram (current) does not indicate new ramp/taxiways. Pilot in Command incorrectly assumed route to taxiway current based on taxi diagram direction sign for Taxiway C as seen from run-up area given impression 'C' can be accepted in either direction. 2) Corrective Action: After uneventful flight; returned to FBO and discussed the issue and how it could be avoided by clear; concise diagram until AFD update.