Not sure on built on site. Give Pat a call, she is super nice and helpful. I bought a 12x36 playhouse with a few add-ons and it took about 10 days. Bit keep in mind my shed eas delivered to the south end of Fresno, CA and it was built in Merced as there was no local shop in Fresno. Been super pleased with my shed. Sorry i couldn't help with the question pertaining to built on site but to give Pat a call or email her.
For me I bought a shed that was already built and it was delivered that week. The owners are super nice, if you email them, they get back to you really quick :)
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