All depends on the authority though. Where I live the Authoritys will let a new Planning Application sit in someones intray on an average for about 8 working days before its even registered. The applications are decided on their target date. That's 10 weeks you have no control of straight away. Before that add a couple of weeks to approve his quote, him to arrange survey, produce existing/proposed you to approve his final proposals (assuming that all goes smoothly), submit to planning. Its easy to see how 12 weeks swallowed up. That also assumes you're happy for him to do the building regs drawings whilst planning is in, remember that unless he's uber confident will be at your risk, if he needs to amend stuff if the planning doesn't go smoothly you may have to pay him to make any changes required depending on his appointment.