You have to do your homework. The county I built in, the regs stated that a structure 255sq ft or over must be anchored and needed all sorts of additional permits. As a result, we only did 250 sq ft on the tiny house. The shell for the tiny house was approx. 11k - not including fixtures/hot water heater/sink/frig/hookups etc. Still under 17k for everything. I think the silver lining was the rural community and super friendly building inspector who could see the structures were solidly built and not thrown together. Got all lumber for the finishing touches from a local mill, stayed away from big box stores as much as possible. For the house, craigslist was my friend - got a beautiful 1930s concrete laundry sink for the kitchen, etsy has some great finds too.
Many builders turned me down because they only build Mcmansions. To keep my cost down, I purchased all fixtures - the mark-up from the contractors is eye-popping and would have put me over budget. I found awesome artistic, unique sinks, faucets, lights on etsy (my chandeliers in the kitchen are handmade stained glass from Turkey) so much cheaper than Home depot/Lowes.
Attached greenhouse is not that more expensive - when the foundation is poured, have them pour that as well with a slope wherever you want the floor drain. I bought the windows/sliding glass door and the roofing material for the greenhouse that was seperate (approx 2k)
Without a very lengthy note, it can be done with research. I went 8k over budget on the house and incorporated alot of expenses from the tiny house into the house construction loan (running septic/electric lines etc)
