a few points
1. in order to build now and apply for retention, you would have to OK it with your engineer / architect (certifier)
2. if you apply for retention, to the letter of the law, you will be applying for permission to retain the whole house as constructed.... (an extension can only be added onto an existing build) therefore fee will be bigger
3. if you submit for planning it will take a min of 12 weeks again 9this may not suit your timescale)
4. do not start and then submit plans... you will be forced to cease construction and seek permission to retain and complete...(you do not want the construction exposed to the elements for 12 weeks with nothing happening)......
i would advise you to consult whoever is certifing your project.. its their call really.....