I would have thought that if the project is of such scale as to require a public works loan then it should have progressed incrementally from concept to consultation, design, scoping, outline planning or pre-app (as appropriate), to a fairly robust project proposal with indicative costing and budget allocation (or precept element) prior to even arriving at the public loan application stage.
I see your problem though, chicken and egg to speak.
Depending on the scale of the project must define what the lead-in timings and precursor actions should be.
If you need a loan it must be a fairly substantial project and a build? The planning pre-app process 'could' serve several purposes - feasibility, heads of terms, design concept and public consultation.
Start the pre-app process Jan, build upon feedback and add detail to the scheme through Spring - Summer, enter budget calculations in the Autumn and submit loan application off the back of that (I don't know what the turn around time is on those loan applications.). Precept demand by following Jan.
If the pre-app advice is less than supportive - bin it, its not viable.