Once you’ve completed the design for your Accessory Dwelling Unit (ADU) and secured the necessary permits, it’s time to begin construction. The process of building an ADU varies depending on the size, complexity, and whether the unit is attached to the main house or detached. While smaller, attached units may take just a few months to complete, larger, detached units can take a year or more. Regardless of the scope of the project, careful planning and project management are essential for a successful build.

Below, we’ll walk through the essential steps involved in constructing an ADU and highlight key considerations to ensure that the process goes smoothly.

1. Step-by-Step Construction Process

Site Preparation

The first step in building an ADU is preparing the site. Whether you’re building a detached unit or converting an existing space (like a garage, attic, or basement), the site must be ready to accommodate construction.

  • Clearing the Land: If building a detached ADU, the site needs to be cleared of any debris, trees, or obstacles that may obstruct construction. This could include leveling the ground and ensuring proper drainage.
  • Excavation and Foundation: For detached units, excavation will likely be required to lay the foundation. A solid foundation ensures the stability and durability of the ADU. Options for foundations include slab, crawl space, or a full basement, depending on local soil conditions and design preferences.
  • Temporary Utilities: If necessary, set up temporary utilities such as water, electricity, and sanitation for construction workers. These are often needed for site offices, tools, and equipment.

Why It Matters: Proper site preparation ensures that construction proceeds without unexpected delays. It provides a stable base for the structure and ensures that the foundation can support the weight of the ADU.

Structural Work

The next phase of construction is the structural work, where the building’s frame, foundation, and exterior shell are constructed. This phase focuses on ensuring the ADU is structurally sound and ready for utilities and interior finishes.

  • Framing: The framing process involves building the skeleton of the ADU, including walls, floors, and roof framing. The type of framing used will depend on your design and materials, with wood framing being the most common option.
  • Windows and Doors: Once the framing is complete, windows and doors are installed. This step is essential for ensuring the ADU is weatherproofed and secure before moving on to the next stages of construction.
  • Roof Installation: The roof structure will be installed after framing, including roofing materials like shingles, tiles, or metal. Proper roofing installation is vital to protect the ADU from water damage and the elements.

Why It Matters: Structural work lays the foundation for the entire ADU. A strong, secure frame is essential for the longevity and safety of the unit. This stage is also when the overall shape and appearance of the ADU begin to take form.

Utilities and Plumbing

Before completing the interior, it’s important to install all utilities and plumbing that will serve the ADU. This step includes electrical wiring, water systems, and sewage connections.

  • Plumbing: Plumbing is one of the most critical parts of any construction project. The ADU will need to be connected to the main water supply and sewage system, which may require digging trenches or connecting to septic tanks or city sewer lines.
  • Electrical Wiring: The electrical system will be installed, which includes running wires throughout the unit and installing outlets, lighting, and power sources. Depending on your design, this may also include dedicated lines for appliances, HVAC systems, and water heaters.
  • HVAC (Heating, Ventilation, and Air Conditioning): If the ADU requires a separate HVAC system, it will be installed during this phase. This includes ductwork, air vents, and any necessary heating or cooling units.

Why It Matters: Proper installation of utilities and plumbing is critical to the functioning of the ADU. Ensuring that these systems are correctly designed and installed helps prevent future problems like leaks, electrical malfunctions, or insufficient heating and cooling.

Interior Finishes

Once the structural and utility work is completed, it’s time to focus on the interior finishes. This is when the space begins to look like a livable unit, with walls, floors, ceilings, and fixtures installed.

  • Insulation: Insulating the walls and ceilings will help regulate the interior temperature and improve energy efficiency. This step is especially important for ADUs built in colder climates, where proper insulation ensures year-round comfort.
  • Drywall and Painting: Once insulation is installed, drywall will be hung, taped, and finished. Afterward, the unit is primed and painted to provide a clean, fresh look.
  • Flooring: Select the appropriate flooring based on the function of the room and your aesthetic preferences. Options include hardwood, tile, carpet, vinyl, or laminate.
  • Cabinetry and Countertops: The kitchen and bathroom areas will be fitted with cabinets, countertops, sinks, and other essential fixtures. This is also when plumbing and electrical fixtures like faucets, lighting, and outlets are finalized.
  • Final Touches: The final phase of interior work involves installing doors, trim, baseboards, and any additional decorative elements to complete the look.

Why It Matters: Interior finishes define the livability and appeal of the ADU. Paying attention to detail ensures a space that feels like home, with functional design and quality materials. These final steps also increase the overall value of the ADU.

2. Managing Permits and Inspections

Securing the proper permits and scheduling inspections are essential aspects of the ADU construction process. Depending on your municipality, you may need several permits to comply with building codes and local regulations. Managing this process ensures that the ADU is legal, safe, and up to code.

Obtaining Permits

  • Building Permits: These permits allow you to begin construction and ensure that the work complies with local building codes. Be sure to apply for and secure permits before breaking ground. This might include specific permits for plumbing, electrical, or mechanical systems.
  • Zoning Permits: Zoning permits confirm that your ADU complies with local zoning laws, including setback requirements, height restrictions, and size limits.
  • Utility Permits: If your ADU requires connections to city water, electricity, or sewage systems, you will need permits for those installations.

Scheduling Inspections

  • Inspection Phases: Inspections typically occur at several stages of construction, including before pouring the foundation, after framing, and before final occupancy. These inspections ensure that the work meets safety standards and is done correctly.
  • Final Inspection: Once construction is completed, a final inspection will be scheduled to verify that the ADU is habitable and meets all code requirements. If everything is in order, the final certificate of occupancy will be issued.

Why It Matters: Ensuring that permits are obtained and inspections are completed according to regulations helps prevent legal issues down the road. Failing to follow the correct process can result in fines, project delays, or the need to redo certain aspects of construction.

3. Managing the Construction Timeline

The construction timeline for an ADU can vary based on the complexity of the project. Smaller projects like converting a basement or garage into an ADU typically take between 2 to 6 months. However, larger detached units may take 9 months to a year or longer.

To manage the timeline effectively, consider the following:

  • Hiring a General Contractor: If you’re not experienced with construction management, hiring a general contractor can help keep the project on track. They can coordinate subcontractors, order materials, and ensure that everything is completed on schedule.
  • Project Milestones: Set clear milestones for each phase of construction (site prep, framing, utilities, interior finishes) and ensure that each stage is completed on time before moving to the next phase.
  • Unexpected Delays: Be prepared for delays due to weather, supply chain issues, or permitting challenges. Build some flexibility into your timeline to account for these issues.

Why It Matters: A well-managed timeline ensures that the ADU is completed efficiently and within budget. Keeping the project on track minimizes inconvenience and helps prevent unnecessary costs.

Conclusion

Building your ADU is a comprehensive process that involves many phases, from site preparation and structural work to utilities and interior finishes. Each step is essential for ensuring that the ADU is both functional and compliant with local codes. Managing the construction timeline and securing the necessary permits and inspections will help ensure the process goes smoothly and that the final result meets your needs and expectations. By staying organized and informed throughout the process, you can successfully build an ADU that enhances your property and meets your lifestyle requirements.