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The 10 Steps to Building an ADU: From a Professional

Accessory Dwelling Units (ADUs) are a popular solution for adding extra housing space on your property. Whether you’re looking to accommodate family members, create a rental opportunity, or maximize your home’s value, building an ADU can be a practical and rewarding project. This step by step guide will walk you through the essential steps to successfully plan and construct your own ADU.

10 Steps to Building an ADU

1 | Understand Your Local Regulations

The first step in your ADU construction project is to research and understand local zoning laws and building codes. These regulations can dictate the size, height, and location of your ADU, as well as whether you can convert existing space or need new construction. Many municipalities now have streamlined rules to encourage ADU development, but you’ll need to confirm what applies in your area. Additionally, building an ADU can be a great way to generate rental income, so it’s important to ensure your plans comply with any rental regulations. Make sure to also review whether there are any homeowner association restrictions. This preparation will save time during the building permit application process.

2 | Establish a Budget

Setting a realistic budget is critical before moving forward with your ADU construction project. Consider the costs of the building permit application, construction materials, labor, and any additional features you want to include, like plumbing and energy-efficient systems. Keep in mind that unexpected costs, such as addressing site issues or changes in design, can arise. It’s wise to set aside extra funds for such contingencies. A well-planned budget ensures that you can complete the project without unnecessary delays.

3 | Design Your ADU

Your ADU construction should be designed to meet your needs while complying with local codes. Decide whether you’ll convert existing space, such as a garage, or build a new structure. Work with a professional designer or architect to create plans that maximize space, energy efficiency, and functionality. Including features like natural light, storage space, and modern layouts will increase the unit’s appeal. Be sure that the final design aligns with what you’ll submit during the building permit application process.

4 | Obtain a Building Permit

Before any construction process begins, you must apply for and obtain a building permit with your local government. Submit your finalized ADU construction plans along with all required documents, such as site plans and architectural drawings, to the permitting office. The permitting process varies depending on your jurisdiction, with approval timelines ranging from a few weeks to several months. During this phase, be prepared to answer any questions or make design adjustments. A granted building permit ensures that your project complies with all safety and zoning requirements.

5 | Hire the Right Professionals

An ADU project is a complex project that requires skilled professionals. Depending on the scope of your project, you may need to hire contractors, electricians, plumbers, and structural engineers. Ensure that your team understands how to properly connect your ADU to the local sanitary sewer system. Conduct thorough research and obtain multiple quotes to find reliable and experienced professionals. Hiring the right team ensures the construction process proceeds smoothly and the unit is built to high standards. This also reduces the risk of costly mistakes.

6 | Prepare the Construction Site

Once permits are in hand, the next step is to prepare the site for construction. If you’re building on vacant land or converting existing space, ensure the area is cleared and complies with safety regulations. This is the stage to address any site-specific challenges like grading, utility connections, or drainage issues. Proper site preparation lays the foundation for a seamless construction process. Any pre-construction inspections may also need to be completed at this point.

7 | Start the Construction Process

With preparations complete, it’s time to start the construction process. Whether it’s extending your property, converting a garage, or creating a completely new structure, this stage involves framing, plumbing, electrical work, and other key steps. Ensure that construction milestones align with your plan and local building codes. Regular communication with contractors and site supervisors will help keep the project on time and within scope.

8 | Focus on Interior Finishing

After the main structure is complete, the focus shifts to interior finishing. This includes installing flooring, cabinetry, lighting, and appliances. Choose materials and fixtures that match the purpose of your ADU, whether it’s for personal use or a rental. This is also the stage to address any final electrical, plumbing, or HVAC installations. High-quality finishing work not only improves the functionality of your unit but also its overall appeal.

9 | Schedule a Final Inspection

Before you can officially use your ADU, it must pass a final inspection conducted by local authorities. Inspectors will review the unit to ensure that it complies with the building permit and all safety standards. This includes checking the structural elements, electrical systems, plumbing, and other key components. Make any necessary corrections if issues are found. Approval during the final inspection is the last major step before occupancy.

10 | Enjoy Your Completed ADU

Once the final inspection is approved, your ADU is ready for use! Whether you plan to use it as a rental property, guest suite, or living space for family, you’ll enjoy the added functionality and value it brings to your property. Take time to review the completed work and address any lingering details with your contractors. Your new ADU is a versatile solution that enhances your property for years to come.

Cost Breakdown for an ADU

Small ADU (200-400 sq. ft.)

A small ADU is ideal for converting an existing structure like a garage or building a compact standalone unit. The cost for a small ADU typically ranges from $50,000 to $100,000, depending on materials and labor. Essential components such as plumbing, electricity, and heating systems make up a significant portion of the expense.

Streamlining the design and keeping finishes simple can help reduce costs, especially if the primary goal is to complement your primary dwelling unit. Despite its smaller size, a small ADU can still offer significant value as a rental unit or flexible living space.

Mid-Sized ADU (400-800 sq. ft.)

Mid-sized ADUs are a popular option for homeowners looking to add a comfortable additional living space. These units generally cost between $100,000 and $200,000, depending on factors such as location, design complexity, and utility upgrades. Mid-sized units often offer the flexibility to include multiple rooms, like a bedroom, bathroom, and even a small kitchen, enabling successful ADU construction to meet specific residential needs.

Converting a structure like an existing garage into a mid-sized ADU may save on foundation and structural costs compared to building from scratch. However, ensuring the structural integrity of the original building is crucial to avoid complications. The complex process of designing and upgrading utilities in mid-sized ADUs adds to their overall functionality while keeping them relatively affordable.

Large ADU (800-1,200 sq. ft.)

A large ADU resembles a small house and is suitable for extended families or high-value rental opportunities. Costs for a larger unit range from $200,000 to $400,000, depending on the customization and materials used. Features like expansive kitchens, multiple bathrooms, and advanced energy-efficient systems are common in this category, but they can significantly drive up costs.

Building a large ADU often involves starting from scratch rather than using an existing structure, making it a larger residential construction project. This requires significant site preparation, engineering, and ensuring the structural integrity of the unit. The complex process of designing, permitting, and building a large ADU adds to the investment but often leads to a greater return on property value and long-term functionality for your household.

Frequently Asked Steps to Building an ADU Questions

What Is the Best Foundation for an ADU?

Choosing the right foundation for your ADU is a critical decision that affects the structure’s durability, safety, and overall cost. The three most common foundation types for ADUs are concrete slabs, crawl spaces, and pier-and-beam foundations. Each option has distinct advantages and considerations depending on your project’s needs and site conditions.

Concrete Slab

This is a popular choice for ADUs due to its affordability and simplicity. A concrete slab provides a stable, low-maintenance base that works well on flat, stable ground. It’s also ideal for areas with minimal risk of frost or soil movement, ensuring long-term durability. However, it offers limited access to utilities underneath the structure, which can be a drawback for future repairs.

Crawl Space

A crawl space foundation is a versatile option, elevating the ADU slightly above ground level. This design provides easier access to plumbing, electrical, and HVAC systems, making maintenance more manageable. Crawl spaces are particularly useful for sloped lots or areas with high water tables. On the downside, they are generally more expensive than concrete slabs and require proper ventilation to prevent moisture buildup.

Pier-and-Beam

Pier-and-beam foundations are another suitable option, especially in areas with uneven terrain or soil that expands and contracts significantly. They lift the ADU above the ground, reducing the risk of damage from shifting soil or flooding. This type of foundation also allows for easy utility access but may require additional structural engineering to ensure proper weight distribution.

What’s One Drawback of an ADU?

One significant drawback of an ADU is the upfront cost of construction and permitting. Even for smaller units, expenses can quickly add up due to design, labor, materials, and the often lengthy permitting process required by local authorities. Unexpected costs may arise from utility upgrades, foundation work, or meeting zoning rules and inspection requirements. While an ADU can add substantial value to your property over time, the initial financial investment may pose a challenge for some homeowners.

Can My ADU Be Bigger Than My House?

Generally, an ADU cannot be larger than the primary residence on the property due to local zoning regulations and size restrictions. Most jurisdictions impose guidelines that limit the size of an ADU to ensure it remains secondary to the main house. These limits are often expressed as a percentage of the primary dwelling’s square footage or a fixed maximum size, such as 800 to 1,200 square feet.

However, specific rules vary depending on where you live. For example, some cities allow slightly larger ADUs in cases where the lot size is significantly large or if the ADU is replacing an existing structure. It is essential to consult your local planning department or a professional contractor to understand the regulations in your area and ensure your project complies with all requirements. This preliminary step can save time and money as you plan your ADU.

Conclusion

Building an ADU is a significant but rewarding investment that can enhance your property’s value, provide additional living space, and generate rental income. Whether you choose a small, mid-sized, or large ADU, thoughtful planning and consideration of design, foundation type, and local regulations are essential for a successful project. While upfront costs and permitting processes can be challenging, the long-term benefits often outweigh these initial hurdles. With the right approach, an ADU can deliver flexibility, functionality, and a valuable addition to your home for years to come.