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In most US jurisdictions a small detached backyard greenhouse does not need a building permit — but almost everywhere still applies zoning rules to it. The International Residential Code exempts one-story detached accessory structures used as tool and storage sheds, playhouses and similar uses where the floor area does not exceed 200 square feet, and many local jurisdictions adopt a lower threshold, commonly 120 square feet. Setbacks, lot coverage, height limits, utilities and HOA rules apply regardless. Confirm with your local building department before you buy a kit.
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Where does the size threshold come from?
From the model code. Section R105.2 of the 2021 International Residential Code lists work exempt from permit, and the first building item is “one-story detached accessory structures, provided that the floor area does not exceed 200 square feet” — 200 square feet in recent editions, raised from 120 square feet in earlier ones. Because states and cities adopt the IRC with amendments, and on their own schedules, the number in force where you live may be 120, 200, or something else entirely, and some jurisdictions require a permit for any structure with a foundation regardless of size.
Practically, that means a common 8 by 10 foot hobby greenhouse (80 square feet) or a 10 by 12 (120 square feet) usually falls under the exemption, while a 12 by 20 (240 square feet) usually does not. If you are close to the line, measure the way your jurisdiction measures — some count roof area or overhangs rather than floor area.
What always needs a permit?
| Feature | Permit usually required | Why |
|---|---|---|
| Over the local area threshold | Building permit | Plan review for structural, snow and wind loads |
| Electrical service — lights, heaters, fans, outlets | Electrical permit | Wet location, GFCI, burial depth and disconnect rules |
| Running water, a sink or a drain | Plumbing permit | Backflow prevention and drainage |
| Gas heater | Gas or mechanical permit | Combustion, venting and carbon monoxide |
| Permanent concrete foundation or slab | Building permit in many jurisdictions | It becomes a permanent structure |
| Attached to the house (lean-to, conservatory) | Building permit, essentially always | It is an addition, not an accessory structure |
| In a floodplain, coastal zone or historic district | Additional review | Separate overlay regulations |
The electrical one catches people out. A greenhouse with a heater, a fan and grow lights is an outbuilding with a branch circuit in a damp environment, and that is permitted, inspected work in essentially every US jurisdiction. The IRC’s list of electrical work exempt from permit runs to five narrow items — decorative lighting, receptacle reinstallation, like-for-like breaker swaps, sub-25-volt wiring and minor repairs — and a new branch circuit to an outbuilding is not among them. Running an extension cord across the lawn permanently is not a legal alternative — it is a shock and fire hazard and most codes prohibit it as permanent wiring. Hire a licensed electrician for the feeder and let them pull the permit.
What does zoning control even when no permit is needed?
- Setbacks — minimum distance to rear and side property lines, commonly 3 to 10 feet for accessory structures. This is the rule most often broken.
- Lot coverage — the total percentage of the lot occupied by structures.
- Height — often 10 to 15 feet for accessory buildings.
- Placement — many codes prohibit accessory structures in the front yard, or forward of the house.
- Easements — utility and drainage easements are typically unbuildable, and a structure over one can be removed at your expense when the utility needs access.
- Distance from the house or other structures, for fire separation.
What should you ask your building department?
One phone call or counter visit answers everything, and departments are generally helpful about this. Ask, specifically:
- What size detached accessory structure is exempt from a building permit here?
- Is a greenhouse classified as an accessory structure, an agricultural structure, or something else?
- What are the setback, height and lot coverage limits for my zoning district?
- Does a permit become required if I add a slab foundation? Electricity? Water?
- Are there design or wind and snow load requirements I need to meet even without a permit?
Write down who you spoke to and when. And note that snow and wind load requirements matter whether or not anyone inspects the build: many inexpensive kits are not rated for northern snow loads or coastal wind, and the anchoring is usually the weak point — see anchoring a greenhouse against wind.
Related: greenhouse sizing and siting for placing it well within whatever setbacks you have, how thick polycarbonate panels should be, how long polycarbonate panels last, and ranked kits in best backyard greenhouse.