What is an ADU? Your Complete Guide to Accessory Dwelling Units

As housing costs rise and cities become more crowded, many homeowners are discovering creative ways to make the most of their property. One of the most popular — and practical — solutions is the Accessory Dwelling Unit (ADU).

An ADU is a secondary, self-contained living space built on the same lot as a single-family home. With its own entrance, kitchen, bathroom, and living area, an ADU offers flexibility for a range of needs — from housing family members and hosting guests to earning rental income or creating a private retreat.

Also known as granny flats or backyard cottages, these thoughtfully designed spaces are perfect for modern living — combining comfort, style, and function in one efficient footprint.

Types of ADU's

There are three main types of ADUs: detached, attached, and interior units. Each comes with its own design considerations and benefits, as well as different zoning and permitting requirements depending on your city or state. We’ll dive into ADU regulations later, but here’s a quick overview of the main types:

Detached ADUs

A detached ADU is a stand-alone structure separate from the main home — often located in the backyard. This type offers the greatest privacy and flexibility, making it ideal for rental units, guest houses, or home offices.

Attached ADUs

An attached ADU is physically connected to the main home, either as a new addition or a conversion of an existing space, such as an attached garage. This option allows for easy utility connections and a seamless extension of the home’s footprint.

Interior ADUs

Interior (or internal) ADUs are created by converting space within the existing home — like a finished basement or attic. This is often the most cost-effective option, as it typically requires fewer exterior modifications.

Each type of ADU has unique advantages, and the right choice depends on your property layout, budget, and long-term goals.

ADU Regulations

The term “Accessory Dwelling Unit” refers to how the space is used, not how it’s built. This means an ADU can take several forms — it might be constructed on-site, factory-built as a modular unit, or even built on a chassis as a tiny home on wheels (THOW).

The rules governing your ADU — including size limits, height restrictions, and setback requirements — are determined by local zoning and building codes, which can vary significantly between cities and states.

For instance, some municipalities require ADUs to be built on a permanent foundation, which means only site-built or prefab modular structures would qualify — not a THOW. Others may allow more flexibility, especially in rural or unincorporated areas.

Before starting your project, it’s essential to review your local ADU ordinance or consult with your city’s planning department to confirm what’s allowed on your property. Understanding these regulations early on will help you avoid costly redesigns or permitting delays.

10 Things to Check Before Adding an ADU to Your Property

Before you dive into designing or building an ADU, it’s important to understand the requirements and potential constraints that could affect your project. A little research upfront can save you time, money, and frustration down the road.

Here are ten key things to review before getting started:

1. Local Zoning Regulations
Confirm that ADUs are permitted in your area and learn what zoning rules apply — including maximum size, placement, and design standards.

2. Building Codes
Your ADU must comply with local building, fire, electrical, and plumbing codes to ensure safety and compliance.

3. Permitting Requirements
Most areas require one or more permits — such as building, zoning, or occupancy permits — before construction begins.

4. Lot Size and Setback Rules
Some municipalities specify a minimum lot size or distance (setback) that an ADU must maintain from property lines or other structures.

5. Parking Requirements
Check whether your city requires additional parking spaces for the ADU. Some areas waive this requirement if your property is near public transit.

6. Homeowners Association (HOA) Rules
If you live in a community governed by an HOA, review their covenants and restrictions — many still limit or prohibit ADUs.

7. Historic District Guidelines
Properties in historic districts may be subject to additional design reviews, material restrictions, or permit steps.

8. Rental and Legal Restrictions
Some municipalities limit how an ADU can be used — for example, restricting short-term rentals or requiring owner occupancy.

9. Utility Connections
Verify whether your existing water, sewer, and electrical systems can support an additional dwelling. You may need to upgrade or install separate meters.

10. Accessibility and Site Access
Ensure your lot is accessible for delivery or construction — especially if you’re installing a modular ADU or tiny home on wheels (THOW). Also, check for any local accessibility requirements for entrances or pathways.

Don’t let a missed regulation become an unexpected roadblock. Research your local requirements early and consult professionals familiar with ADU development in your area.

5 Benefits of Accessory Dwelling Units

An Accessory Dwelling Unit isn’t just a smart way to add living space — it’s an investment that can deliver long-term financial and lifestyle rewards. Whether you’re thinking about family needs, future flexibility, or generating income, an ADU offers meaningful advantages for homeowners.

Increased Property Value‍

Adding an ADU expands your livable square footage and can significantly boost your home’s resale value, especially in markets where flexible housing options are in high demand.

Rental Income Potential

Turn your ADU into a steady income stream by renting it out long-term or seasonally. The extra revenue can help offset your mortgage or fund other property improvements.

Flexible Use

An ADU can adapt to your lifestyle — serving as a guest suite, home office, art studio, wellness space, or short-term rental. The possibilities are endless.

Creative Freedom

Design a space that’s uniquely yours — a restorative sanctuary, art studio, or spa-inspired getaway. ADUs allow you to explore personal expression while enhancing your property’s character and functionality.

Versatility Over Time

As your life evolves, your ADU can evolve with it — offering space for aging parents, college-age kids, visiting guests, or future rental income. It’s an investment that can grow and adapt with your changing needs.

Ready for a Turnkey ADU?

Whether you’re looking to create additional space for your family, generate rental income, or add a dedicated office or studio, an ADU can be a beautiful and practical solution that enhances both your lifestyle and your property’s value.

At Wind River Built, we specialize in modular homes and park models that serve as turnkey solutions and are designed to blend seamlessly with your home and surroundings. Through our collaborative design-and-build process, we craft spaces that reflect your vision while minimizing on-site disruption and delivering exceptional quality in a fraction of the time of traditional construction.

Your ADU should be more than just an addition — it should be a thoughtful extension of how you live, work, and connect at home.

Whether you’re expanding your property or developing a new project, our team can help you design and build an ADU that’s beautiful, functional, and built to last. 

Reach out to us today and get started!

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