This post is one step in a live build. The Post Falls ADU tracker shows where the whole project stands today.

July 6, 2026

Every question an Idaho ADU builder needs answered. What did I miss?

By Russell Mann — Post Falls, Idaho

This page is only useful if it's right. If you find a mistake, tell me and I'll fix it.

I sat down this week and wrote out every question an Idaho ADU builder actually needs answered before they spend a dollar. Not the questions a city handout answers. The questions you ask yourself at the kitchen table, in order.

Four buckets, and the questions under each one:

Am I even allowed to build one?

• Is my property in a city over 10,000 people, covered by Idaho's new ADU law, or a small town or unincorporated county running its own rules?
• Does my city allow an ADU by right, or does it require a discretionary hearing with no guarantee?
• If my city's code hasn't caught up to the new state law yet, can I act on the state law today anyway?
• If my city is right at the population line, which count actually controls: the decennial census, or a more recent estimate?

What can I actually build?

• What's the max size, in square feet or as a percentage of my main house?
• Is there a bedroom limit?
• How many ADUs am I allowed per lot?
• Does my city allow a brand-new detached ADU, or only a conversion of existing space like a garage, attic, or basement?
• Does the ADU or the main house have to be owner-occupied?
• Can I rent it short-term, or only long-term?
• What are the setback, height, and parking rules for my zone?
• Are there timing restrictions, like a waiting period after my home's certificate of occupancy, or a rule tied to how built-out my subdivision is?
• Is there a deed restriction that keeps me from ever selling the ADU separately?

What does it cost, and how long does it take?

• What's the permit fee, and how long does review actually take?
• Can an ADU share my existing water and sewer connection, or does it need its own separate hookup fee?
• Does my existing electrical service have spare capacity, or does it need an upgrade?
• If I'm not on city service, is septic capacity actually checked before the building permit is issued, and does my lot even have a spare allotment?
• If I use one of the free pre-approved plans, how much site-specific engineering is still left to do before I can actually permit it?
• Start to finish, how long does a permit-to-occupancy timeline realistically run?

How do I pay for it, and what happens to my existing home?

• What financing actually works for this: cash, HELOC, construction loan, something Idaho-specific?
• Does adding an ADU trigger a property tax reassessment?
• Does it change my homeowner's insurance?
• Are there pre-approved free plans, or does it have to be custom?

That's about twenty-five questions. I don't have a clean answer to all of them yet. Some I do, mostly the zoning ones. Some I'm actively chasing. I emailed Post Falls this week about the utility hookup question. Some I haven't touched at all: financing, tax, insurance.

Now the actual ask. If you've built an ADU in Idaho, or you're trying to right now, what question is missing from this list? What did I not think to ask? Send it through the contact form on the homepage, or just email me. I'll add it to the list and go find the answer.