ᐅ Construction drawings, structural calculations, and building permit application for a self-built garage.

Hello. We own a small house and would like to have a prefabricated garage installed at the end of our garden. I have already obtained a few quotes, which are fine. However, since the access path to the garden is quite narrow and the truck would have to navigate a turn, no representative can guarantee with 100% certainty that the installation will be possible. So, we have been considering building the garage ourselves. I also have a friend who is a mason and can do the work. But how can I get an affordable construction plan with completed structural calculations for the building permit / planning application for a self-build? Does anyone perhaps have a template drawing, for example, for a 6-meter (20 feet) long by 3-meter (10 feet) wide garage that I could submit? Or is there maybe an architect who could help me obtain the necessary documents? Thank you.