ADU
What Is an ADU? Accessory Dwelling Units (ADUs) Explained in Plain Words
Sep
ADU stands for accessory dwelling unit. It is a second, smaller home on the same lot as your house, with its own kitchen, bathroom and entrance. Here we explain the types, local rules and prefab delivery costs.
You may have seen the letters on a city notice, in a real estate listing or on a neighbor’s building permit. ADU stands for accessory dwelling unit. It is a second, smaller home on the same lot as a single-family home, with its own kitchen, bathroom, bedroom and front door. We sell prefab backyard homes that arrive finished on a truck, so we get this question every week. Here is what counts, how the different types work, what zoning and permit rules generally say, and what one costs delivered.
What the letters stand for, and the other names for it
Accessory dwelling unit is the planning term. You may also hear granny flats, in-law units, casitas, backyard cottages, carriage houses, guest houses, accessory apartments and secondary units. Real estate listings use “ADU home” or “ADU unit.” These names all describe a complete small home that is secondary to the principal dwelling, or main house, on the property. Accessory means it goes with the main house. In most places, you cannot sell it on its own as a separate parcel.
What counts as an accessory dwelling unit
A shed with a bed in it is not an ADU. The unit needs four things: a place to sleep, a full bathroom, a kitchen or at least a kitchenette where you can cook a meal, and its own entrance. It must also be a legal residential unit. That means a building permit, inspections and the same code requirements as any house. These requirements set it apart from a pool house, workshop or accessory structure with no plumbing. Floor area runs from about 200 square feet for a studio to 1,200 square feet for a two-bedroom unit. Most cities cap the size somewhere in that range.
Why homeowners build a second home
- Parents or grandparents. This is the first of the benefits of ADUs: family stays close without sharing a kitchen. A one-level unit can sit a few steps from the back door. This is the classic in-law suite, and the reason most people house family members this way.
- An adult child. They have a place of their own on the property while they save for the next step.
- Rental income. A long-term tenant can live in a detached unit with its own door and address. In many cities, these housing units are the reason the rules were loosened. Backyard rental units add to the housing stock and the supply of affordable housing without new land.
- A home office or studio. You have a real building with a bathroom and a kitchenette, away from the noise of the house.
- Downsizing in place. Some property owners move into the smaller home and rent out the main house. Either way, adding an ADU to your property adds a second livable home.
Types of ADUs
Detached ADUs
This is a separate building in the backyard or side yard, with no wall shared with the single-family home. It is the ADU type most people picture and the type we sell. This type of ADU gives you the most privacy and has its own entrance. You can place it wherever zoning allows. A separate building also suits prefab construction. A crane can lift the finished unit onto a foundation without touching the existing house.
Attached units
An attached unit is an addition to the main house that shares at least one wall and has its own outside door. It can cost less to plumb and wire because those services are close. Construction takes place on site over months, though, and changes the look of the house. A converted basement or a suite made from existing rooms is usually called an interior accessory dwelling unit. It works the same way.
Garage units
This can be a garage converted into living space or a new unit built above a garage. A conversion reuses the walls and slab, but you give up the parking. Many older garages need a new foundation, insulation and plumbing to pass as a dwelling. We do not sell garage conversions, but the same rules about a kitchen, a bath and a permit apply to them.
Junior units
A junior accessory dwelling unit is a California term for a unit of no more than 500 square feet inside the main house. It usually comes from a bedroom or an attached garage and has an efficiency kitchen. Sometimes it shares a bathroom. The distinction is that a junior unit stays within the existing walls. It is a permit category rather than a building you buy.
Zoning, permits and the local rules
Land use is local, so ADU regulations depend on your city and sometimes your county. Over the last few years, many states, including California, Oregon, Washington and Utah, have passed some form of ADU law to make it easier to build an ADU on a single-family lot. Thousands of cities have followed by changing their zoning code to allow ADUs in residential areas. When you check the zoning regulations, your planning department asks the same questions everywhere:
- Is it allowed in your zoning district? Most single-family lots now qualify, but historic districts and homeowner associations can have their own limits.
- How big can it be? Local zoning sets a maximum floor area, often tied to the size of the main house. It also sets a maximum height.
- Where can it sit? Setbacks, the required distances from the property lines and the principal dwelling, decide where it can go in your yard.
- Parking requirements and utilities. Some cities still want a parking space, while many have dropped that requirement. Water, sewer and power usually connect to the main house, sometimes with a separate meter.
- Who can live there? Owner-occupancy rules and short-term rental rules vary the most from place to place.
- Property taxes and grant programs. The new ADU is assessed, and your property taxes rise by its value, not the whole property’s. Some states and cities have run grant programs for ADU development. Your planning department knows whether yours has a grant program.
The building permit application is your responsibility, but you do not have to work it out alone. Before we take an order, Irontown runs a feasibility review of your address, the lot and the route a truck would take. They tell you whether the unit can go in before you have spent anything beyond the review fee. If the answer is no, you stop there.
How a prefab unit is built and delivered
Our units are built by Irontown Modular in a factory in Spanish Fork, Utah. Crews frame, wire, plumb and finish each home indoors. The kitchen, bathroom, flooring, windows and heating are already in when it leaves. Construction follows the same residential building code as a house framed on site. The home leaves the factory as one or two sections and travels by truck to your lot. A crane places it on the foundation your contractor poured. A crew joins and seals the sections and connects the utilities. Because it is a house on a permanent foundation, it is appraised and financed like a house, not like a manufactured home.
The timeline has three steps. The feasibility review takes one to two weeks and comes with a budget. Pre-construction takes two to three months for the site visit, your finish selections and the stamped plans. Then allow 12 to 16 weeks to build, ship and set the unit. Delivery and the crane are included for the first 250 miles from the factory (650 miles in California), and $12.75 a mile beyond that. Irontown is certified for Utah, Idaho, Montana, Wyoming, Nevada, Colorado, Arizona, Oregon and California.
Our ten prefab units, by size
Every unit below is a detached accessory dwelling unit with a full kitchen and a full bath. Prices are for the Designer finish, delivered and set within the first 250 miles.
| Unit | Size | Beds / baths | Price |
|---|---|---|---|
| Mysa 500 | 497 sq ft | 1 / 1 | $137,800 |
| Cabana 500 | 497 sq ft | 1 / 1 | $179,700 |
| Cabana 500, ready to ship | 497 sq ft | 1 / 1 | $160,000 |
| Sledhaus 500 | 580 sq ft plus loft option | 1 / 1 | $174,200 |
| Cabana 700 | 638 or 718 sq ft | 2 / 1 or 2 / 2 | $170,900 |
| Torrey Flats | 696 sq ft | 1 / 1 | $188,200 |
| Morada 800 | 793 sq ft | 1 or 2 / 1 or 2 | $236,100 |
| Morada 1000 | 989 sq ft | 1 or 2 / 1 or 2 | $240,700 |
| Eagle Landing 1000 (Utah only) | 1,000 sq ft | 2 / 2 | $221,100 |
| Rabbit 1000 (two story) | 1,002 sq ft | 2 / 2 | $232,300 |
The Cabana 500 is a good place to start. It has a living room, a full kitchen, a private bedroom, a full bath and a covered front porch in under 500 square feet. The two-story Rabbit 1000 fits a two-bedroom unit on a narrow lot. For anything under 400 square feet, our prefab tiny homes are built to the park model code and cost less. Whether one counts as an ADU depends on your city.
What to do next
Pick the plan closest to what you need and request a quote from its page. We are an authorized Irontown dealer with a 100 percent price match promise. Delivery and the crane are included for the first 250 miles. A real person checks your lot and route before you commit to anything. Send us your address and the plan you like, and we will tell you straight whether an ADU fits.
