Wouldn't it make more sense to call City Hall and ask them? None of us even know which city you are in.

Check your zoning requirements. In MA, a building permit isn't required if the greenhouse is covered with plastic sheeting. It is also exempt from setback requirements as long as it doesn't pose a public safety issue. This info should be available online.

In my area, the amount of setback is less for an accessory building under 200 sqft -- such as a greenhouse -- than it is for other, larger structures. Bottom line, follow the advice above and contact your local permitting authority. It can't be any worse than what you are assuming!

In some municipalities, it is the type of foundation that determines if a building permit is required or not. A "permanent" foundation is a poured concrete slab; a "temporary" one is pressure-treated wood sills with gravel and/or pavers for flooring. I didn't need a building permit but I did need an electrical permit since I brought in 220v for a sub-panel.

TomSlawson has the best answer as other municipalities do not govern where the OP lives, wherever that is. It would be a real bummer if a temporarty structure like a low tunnel/hoop house required a permit though.

Our greenhouse didn't require a building permit because it was not over 120 square feet. Electrical was a different matter. Not sure about the setback requirements as it wasn't an issue for us.

I agree about asking if the foundation type would make a difference. Our local tax appraisal district visited to see if our greenhouse was a taxable improvement, and ruled it was not because it has a temporary foundation as apg4 described (wooden sills and gravel/pavers inside.)

Here, we sometimes have better luck by driving downtown and asking questions face to face. We've gotten some wonky info over the phone in the past, from folks who seemed happy to just "make things up" (not what you want before you build!)

My city requires a permit for any sort of construction that exceeds 12 feet (in any direction). Even if you are remodeling and replacing 12 feet of interior wall without touching the support system you need a permit. When I sent in my permit request it took a week of back and forth for them to decide that I really did need one. Their biggest issue was whether the general public would be stepping into my plastic covered hoop house.

The fact that it was metal hoops mattered. The fact that it had plastic sheeting for walls and ceiling didn't. How the hoops were anchored mattered, who installed them didn't. I also had a 10 foot set back requirement but the inspector never measured anything.