I believe that before winter your shell structure in your situation probably won't be sealed anymore, so that the "interior trades" could continue afterwards. That means you will likely have a winter break in between. Construction start in June or July and everything will be fine, no question! But:
were just with the general contractor this week and discussed the schedule. Completion within the next four to six weeks would still fit within this year.
The general contractor is only waiting for your signature, he would never say "hmm, well, it might be tight this year" because the risk that you run to a competitor if everything is going to take longer anyway is way too high.
Just a quick note from our experience:
Land purchase --> land registration took over 3 months because the key person there was sick. Isn't there a deputy? Apparently not! I mean, WTF?
But that's all preliminaries; the actual construction start was also not as easy as expected:
We also had a development plan with an exemption instead of a building application, but: the planning went in several phases: tender planning (you have completed that), approval planning for construction notification was somewhat tougher because there were issues like emergency exits vs. electric shutters = additional costs and such stories on the table. Then comes the detailed planning or execution planning, structural engineering, ventilation system planning, drainage plan, roof truss plan and who knows what else. All in iterative loops, since for example the statics required a thicker slab and reinforcements, which slightly affected the floor plan, which in turn changed the statics and so on and so forth. At times it felt somewhat slow to me. Then the surveyor has to do the rough survey/fine survey on call, more time might pass here as well... time that in the ideal process of your general contractor might be a bit tight.