What Should You Know Before Building an ADU?

Before building an accessory dwelling unit, or ADU, check local zoning and building rules, confirm your budget, evaluate the site, and decide how the space will fit your household or rental plans. An ADU can add flexibility to a property, but the project works best when the design, permits, utilities, and construction plan are clear from the start.

That practical planning can open up more possibilities than you might expect, helping you create a single property that does the work of two.

When a parent needs a place to live, an adult child needs a place to work, or a rental property needs to do more than one job, an ADU can make the whole setup work a little better. Accessory dwelling units, or ADUs, are secondary homes built on the same lot as a primary residence.

They can serve a lot of different purposes. Some are used for aging parents, adult children, or guests. Others become home offices, art studios, or rental units that help offset a mortgage.

In areas where housing costs are especially high, that extra income can make a real difference. ADUs are especially common in places with strong demand for both short- and long-term housing. Once you know what an ADU can do, the next step is figuring out the type, the rules, and the planning that go into building one.

Key Takeaways:

• An ADU is a secondary living space built on the same lot as a primary residence.
• Common ADU types include detached units, attached units, garage or basement conversions, and junior ADUs.
• Local zoning, size limits, setbacks, permits, utility capacity, and rental rules can affect what you are allowed to build.
• A realistic budget should include labor, materials, design fees, permits, utility connections, and a contingency fund.
• A simple footprint, efficient kitchen and bathroom layout, and pre-approved plans where available can help control costs.
• Storage can help keep tools, furniture, boxes, and approved household belongings out of the way during construction.

What Is an ADU?

A legal ADU usually includes a kitchen, bathroom, and separate entrance. Size requirements vary by location, and some units may be smaller than others depending on local rules.

There are a few common types:

• Detached ADUs: Separate from the main home, such as a backyard cottage, casita, or carriage house.
• Attached ADUs: Connected to the primary home and often called granny flats or in-law suites.
• Converted ADUs: Built from existing space, such as a garage or basement.
• Junior ADUs, or JADUs: Smaller units, often built within the walls of the main house.

The right type depends on your property, budget, household needs, and local requirements.

Check the Fine Print

Rules and regulations vary widely by state, city, and region, so it’s important to check with your local planning and building departments before you start. Setbacks, height limits, size restrictions, parking requirements, zoning rules, and rental regulations can all affect what you’re allowed to build.

Skipping that step can lead to fines, stop-work orders, or costly changes later.

It also helps to look at the practical side of the project early. A site study can reveal issues like utility capacity, soil conditions, drainage, and access before construction starts. That gives you a better chance of protecting the main home and avoiding expensive surprises later.

Before you move forward, make sure you have a clear budget that includes:

• Labor and materials
• Design fees
• Permits and inspections
• Utility connections
• Site preparation
• Landscaping or access changes
• A contingency fund for unexpected issues

For a new build, a 10% to 15% contingency fund is often recommended. For an attached or converted ADU, a larger buffer of 10% to 20% may be more realistic if you run into termite damage, water damage, plumbing issues, electrical upgrades, or other hidden conditions.

Getting several bids is another important step. Since labor and materials are usually the biggest expenses, take the time to check references and, if possible, visit completed projects. If you’re hiring a contractor, look for someone with direct ADU experience.

Measure Twice, Build Once

A good ADU design should balance function, comfort, and privacy. Storage, natural light, and sound control all matter, especially if the unit is attached to the main home.

Think through:

Window placement should be thoughtful for both the ADU and the primary residence. Soundproofing can be especially important if the spaces share walls.

Plan Before You Pour

There are a few ways to keep costs under control without cutting corners.

Smaller square footage can reduce framing and labor costs. A simple rectangular footprint and a single-pitch roof can also help keep construction more straightforward. Going with all-electric appliances may eliminate the need to run gas lines.

Placing the kitchen and bathroom along one wall can reduce plumbing costs by creating a more efficient wet wall. If you’re building in a market where ADUs are common, check to see whether your local building department offers pre-approved plans. Starting with one of those can save time and reduce design costs.

You can still personalize the unit with your own finishes. The layout may be efficient, but the space can still feel like home.

Make Room to Grow

ADUs can add flexibility, function, and growth to a property when they’re planned well. They can create space for family, guests, work, creativity, or a future rental plan—depending on what local rules allow.

If you need room for tools, materials, furniture, or household items while you build, Public Storage can help keep approved belongings out of the way until the project is done. Use the Public Storage size guide to compare unit options for your project needs.

A well-planned ADU can turn one lot into a lot more than one home.

One property. Two homes. Unlimited possibilities.

FAQs: Building an ADU

What Is an ADU?
An ADU, or accessory dwelling unit, is a secondary living space built on the same lot as a primary residence. It may be detached, attached, converted from existing space, or built as a smaller junior ADU, depending on local rules.

Do I Need a Permit to Build an ADU?
Most ADU projects require permits and must follow local zoning, building, utility, setback, height, and size requirements. Check with your local planning and building departments before starting.

How Much Does It Cost to Build an ADU?
ADU costs vary based on size, location, site conditions, labor, materials, utility connections, permit fees, and design choices. Build a budget that includes a contingency fund for unexpected construction issues.

What Is the Difference Between an Attached and Detached ADU?
An attached ADU shares a wall or structure with the main home. A detached ADU is a separate building on the same property, such as a backyard cottage or casita.

Can Storage Help During an ADU Project?
Storage can hold approved tools, furniture, boxes, and household belongings while construction or a conversion project is underway. This can keep work areas, garages, and rooms clearer during the build.