An ADU for adult children is a small, separate home on your lot for a grown son or daughter. They get their own front door, kitchen, and bathroom. You get them close by without anyone moving back into their old bedroom.
If your kid is working, saving, and still priced out of the local market, it’s one of the few fixes a parent can build. Here’s what to settle before you do.
Why an ADU for adult children works
The usual options aren’t great. They move back into the house and everyone loses privacy, or they rent across town and the money you’d have helped with goes to a landlord. A backyard ADU sits between those two.
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It’s a real home. A kit-built ADU goes on a permanent foundation, gets inspected like any house, and has a full kitchen and bath. It isn’t a trailer or a converted garage.
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It stays yours. In most cases the ADU is part of your property and can’t be sold off on its own. The money you put in stays in your lot’s value.
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It has a second life. When your kid moves on, the same building can house you, a parent, or a tenant. Plenty of families plan it that way from day one, and our guide to an ADU for aging parents covers that side.
What to settle before you build
Whether your lot allows it
Both states have made backyard homes easier to permit. We’ve covered Idaho’s ADU law, SB 1354, and the Washington ADU law in detail. Your city still sets setbacks, height, parking, and maximum size, so call your planning department before you pick a model.
The size
One person or a couple doesn’t need much, but they do need a real bedroom, a real kitchen, and storage. The 799 sq ft Cottage kit home is the model families look at most for this, and it usually fits under city size caps. Check your cap first.
The money
Have this conversation out loud, before the foundation goes in:
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Who pays for the build? It’s on your property, so the loan is normally yours. Our guide to kit home financing covers the options.
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Is there rent? If you charge rent, it’s rental income, and renting to family below market rate changes how the IRS treats the property. See IRS Topic 415 and ask your tax preparer.
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Who pays utilities? Decide whether the ADU shares your meters or gets its own. It’s cheaper to choose before the trenches are dug.
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What happens later? If you have other kids, or the house will be sold someday, write down what everyone expects.
Privacy
Placement matters more than square footage. Give the ADU its own path from the street, face its door away from your kitchen window, and give it its own patch of outdoor space. A grown adult who has to walk past your back door to get home doesn’t really have their own place.
How the build runs
It follows the same order as any house: plans, permit, site work and foundation, kit delivery and framing, then trades and finishes. Framing is the part a kit speeds up. Permitting and trade schedules run at local pace. We walk through each stage in our guide to an ADU build in Idaho and Washington.
If your kid is handy, some of the work can be theirs. Owner-builders often take on finish work, while plumbing, electrical, and mechanical go to licensed trades.
ADU for adult children FAQ
Can my adult child live in an ADU on my property?
Yes, where your lot can be permitted for one. An ADU is a legal dwelling, and family is the most common reason people build them.
Can my child buy the ADU from me later?
Usually not on its own. An ADU is normally tied to the main house and the lot. If ownership matters to your family, talk to a real estate attorney before you build.
What if they move out?
Then you’ve got a finished second home on your lot. Many owners switch to a long-term tenant, and our guide to ADU rental income shows how to run those numbers.
The bottom line
An ADU for adult children gives your kid a real home and keeps the investment in your own property. Sort out the lot rules, the size, and the money conversation first, and plan for what the building becomes after they leave.
If you’re thinking about one in Idaho or eastern Washington, give us a call.
