In the United States, the commonly accepted maximum size for a tiny home is 400 square feet, Appendix Q of the 2023 International Residential Code (IRC) deals specifically with tiny houses. There it defines a TINY HOUSE as "A dwelling that is 400 square feet (37 m2) or less in floor area excluding lofts."
Earthship homes have developed plans for tiny homes. The design I'm looking out would be built similar to this layout. There is a green house along the front that works for a small garden and passive heating. At the back end of the house where the bio mass heat storage is located with the cistern are some long underground tubes that run some 50 ft buried around 5 feet under the ground so that air vented thru them into the home will provide 55 degree F air for A/C. When the roof vents are opened at the front of the home at the top of the roof the warm/hot air will draw the cool into the home as it rises out. Solar panels will charge a bank of batteries and an inverter will convert it to AC electric for domestic appliances etc.Earthship homes are designed to be sustainable and self-sufficient, often incorporating natural and recycled materials, passive solar design, and energy and water conservation systems. They are known for their unique construction, which may include walls made of old tires packed with dirt and roofs that can be used for rainwater collection and food production.
wow, I guess @eDJ_ has gave you all of the answer in detail lol. maybe one thing that I want to add, which is, don't forget the documents or certifications. I mean, this is huge for RV parks. Many parks require homes to be RVIA certified for safety and to comply with zoning. It impacts insurance, financing, and legalityMany tiny homes are built on trailers of various sizes. My grandson wants to build one and live in an RV park in it. He had helped build big family homes, so he had the experience.
What are your best bits of advice for him?