Custom ADU: Your Ultimate Guide

What is a Custom ADU?

A custom ADU is an accessory dwelling unit designed entirely to fit the homeowner’s specific needs, including a personalized floor plan and chosen finishes. In contrast, semi-custom ADUs use pre-approved plans for the layout but still allow homeowners to select the floors, cabinets, countertops, and other finishing details of their choosing. Since an ADU is a secondary home on the lot of an existing primary dwelling, going custom means you can match the buildings as closely as possible aesthetically.

Both custom and semi-custom ADUs are built entirely on-site, meaning they’re built directly on a homeowner’s property. This is different from manufactured or prefab ADUs that are built off-site in a factory-controlled setting and then shipped in pieces or as a whole unit to the property.

A custom ADU is fully tailored to your lifestyle, offering the freedom to create a seamless extension of your home.

Benefits of Building a Custom ADU

By opting to build a fully custom ADU in the San Diego area, you not only maximize your investment to get the most bang for your buck in rental income and property value, but you also create an ADU that meets your family’s exact needs as a granny flat, guest space, or rental unit. This can also allow you to stay in your existing home for longer down the road.

Investing in a custom ADU can significantly enhance your property value while creating a versatile space tailored to your needs.

Long-Term Investment

While custom ADU projects do tend to have higher construction costs than other ADU builds, these additional costs can yield a greater return on investment.

If you’re looking to use your ADU as a rental unit, a custom space may allow you to charge more in rent per month than a prefab unit. Consider this project of ours from Encinitas: The ADU contained 2 bedrooms, 2 bathrooms, and clocked in at 1000 square feet. It cost just under $400,000 to build, but when you consider that the final construction loan payments were $3,000 a month and the couple was able to charge over $3800 in rent per month, that investment was well worth it. These homeowners will pay off their loan in just over 4.5 years, at which point their rent becomes pure additional income.

Even if you aren’t planning on renting your ADU and choose to use it as an in-law suite, guest house, or extra living space instead, going custom can increase your overall property value more than a non-custom build. By some estimates, it’s as much as 35%.

Regulatory Advantages

Many homeowners we meet are intimidated to undergo the permitting process, especially for a custom ADU. But you should know that California laws are in place to protect homeowners pursuing a custom ADU and to make the construction process as smooth as possible.

One of the most relevant laws is AB 2221, which says that building permits must be approved or denied within 60 days of application. This expedited permitting process prevents delays, and with it, most of our custom builds at Better Place are completed within only 8-12 months.

Design Freedom

Going custom does involve some design fees that you don’t see with prefab ADUs, but homeowners often find these fees are more than worth it to bring their vision to life.

When you build a custom ADU, you can design every single detail from the layout to the flooring, to the doorknobs and cabinet pulls. And when you work with an experienced ADU construction company during the design phase, you can even save money on your finishes, as they’ll have the expertise to help you achieve the high-end look you desire without using only high-end (read: high-priced) materials.

Note that designing an ADU isn’t quite the same as designing a single-family home. We’ve compiled some of our best ADU interior design tips for you to read here.

How Much Does it Cost to Build a Custom ADU in California?

The costs of building a custom ADU in San Diego County can vary. When it comes to detached ADUs between 500 and 1200 square feet, we tell homeowners to estimate between $300,000 and $450,000.

Understanding the cost breakdown of a custom ADU, including foundation work and permits, helps ensure your project stays on budget.

Custom vs. Prefab Cost

Prefab ADUs don’t include all the additional costs associated with a custom unit, such as design services and finishing costs, since these elements are pre-determined by the prefab ADU companies.

That said, prefab ADUs can have hidden fees. Transportation fees for delivering the finished unit to the site are an additional expense that custom ADUs don’t have. Prefab units also still require foundation work, which is often not included in the base price. Since every ADU, whether custom or prefab, requires a stable foundation, this is an essential cost that must be factored into your budget.

If cost and budget are a primary concern, an attached ADU or a garage conversion may be a more cost-effective option. These types of builds require fewer materials, and some structural components, like exterior walls and foundations, are already in place.

Common Design Options for Custom ADUs

One of the best ways to envision your custom ADU is by reviewing pre-approved ADU plans. These plans offer a cost-effective way to customize your space without starting from scratch.

Studio ADU Floor Plans – Ideal for home offices, guest houses, and apartments under 400 square feet.

1-Bedroom Floor Plans – Versatile layouts ranging from 400 to 600 square feet, perfect for aging parents or rental use.

2-Bedroom Floor Plans – Designed for comfort and rental potential, typically ranging between 500 and 750 square feet.

3-Bedroom Floor Plans – Great for small families or long-term multigenerational living, usually starting at 1000 square feet.

4-Bedroom Floor Plans – Maximizes ADU potential, turning your backyard unit into a second single-family home with up to 1200 square feet.

While most ADUs are single-story, many cities in San Diego County allow for two-story structures. A two-story ADU is a great way to maximize square footage while minimizing the overall footprint on your property.

Cost Breakdown

Several factors contribute to the cost of building a custom ADU, including permit fees, design and architectural costs, foundation work, roofing and siding, utility connections, and interior finishes. In addition to these major expenses, other construction costs may include landscaping, kitchen appliances, bathroom fixtures, and interior finishes such as flooring, lighting, and cabinetry.

Below is a breakdown of some of the most significant costs associated with a custom ADU:

What to Look for in Custom ADU Builders

When looking for a construction team to take on your ADU project, it’s important to look for one that’s experienced in ADU builds specifically, not just residential construction. ADUs have different zoning laws and building codes that must be followed. If not, it can lead to costly delays and fees.

Also, look for a company that can be with you from design to construction. This will ensure a smooth build and can even lead to some cost savings in design fees and overall labor costs.

Lastly, before you hire any ADU contractor, be sure to meet with them in person to discuss your vision. Look for a contractor that’s transparent in their communication style, has positive reviews, and has a commitment to quality (like Better Place Design & Build!)

Partner with an experienced ADU builder who understands local regulations and delivers both design and quality.

Ready to Build Your Custom ADU?

At Better Place Design & Build, we’ve never met a property owner who’s regretted going custom. Our dedicated team of designers and builders will walk you through every step of the custom building process to ensure it gets completed on time and within your budget.

Every successful project starts with a free initial consultation. Contact us today to schedule yours.