The honest answer is that it depends on your city, and anyone who tells you otherwise on the internet is guessing. What we can give you is the shape of the question, so the ten minute phone call to your building department is quick and useful.
What usually triggers a permit
Across most jurisdictions the same handful of factors come up:
- Footprint. Many places exempt small freestanding structures below a threshold, often somewhere between 100 and 200 square feet. Above it, you file.
- Attachment. The moment a structure is fixed to your house it usually becomes part of the building rather than a yard object, and the rules tighten.
- Foundations. Anchoring into a poured footing is treated differently from posts sitting on a patio.
- Height. There is normally a ceiling above which you need approval regardless of footprint.
- Setbacks. How close you can build to the property line. This one catches people on narrow city lots more than anything else.
- Electrical. Anything wired, saunas especially, usually needs an electrical permit and inspection even when the structure itself does not.
The other approvals people forget
A building permit is not the only gate. If you are in an HOA, a condo, a co-op or a historic district, there is often a separate architectural review with its own timeline and its own opinions about what your backyard should look like. That process can take longer than the city one.
Renting? You need your landlord's written consent before anything gets anchored to anything.
Saunas specifically
Two layers. The structure itself follows the rules above. The heater is an electrical matter, and most units want a dedicated circuit. Even where the sauna is small enough to be exempt as a structure, the wiring generally is not, and an electrician who has done one before is worth the call.
How to get your actual answer
Call your local building department with four things ready: the footprint in feet, the overall height, whether it will be freestanding or attached, and whether it will be anchored to a foundation. That is enough for them to tell you yes or no on the spot in most cases.
Ask about setbacks while you have them on the phone, because it is the one that quietly changes what you can buy.
Why this is worth doing first
A pergola is a freight delivery on a pallet. Discovering a permit problem after it lands in your driveway is expensive and slow. Discovering it beforehand costs a phone call and occasionally means sizing down one step, which is a much better outcome.
If you are unsure what to ask, tell us what you are planning and we will help you frame the question.