ADU Permits Explained (Part 2): What Permits Are, Why They Matter, and How to Apply

By: Adrienne Norris

After you decide to build an accessory dwelling unit (ADU), the next phase is usually permits. At first, the process can feel complicated. Once you understand what permits are required, how applications work, and what timelines look like, it becomes much easier to navigate.

Part 1 of this series covered the big-picture questions like state rules and property eligibility. In Part 2, we focus on building permits: what they cover, why they are important, and how to move through the process.

Building Permits 101

Before construction begins, most homeowners need one or more building permits. These approvals allow the project to move forward under local building codes, zoning regulations, and safety standards. Permits help ensure the structure is safe, compliant, and formally documented, which can also protect the investment if you sell or refinance later.

Do You Need a Permit to Build a Backyard ADU?

In most cases, yes, especially for a habitable space like an ADU. Even if you are using a prefab kit or panelized system, permits are typically required to:

  • Confirm the project complies with zoning rules
  • Validate foundation, electrical, and plumbing requirements
  • Ensure the design meets safety and dwelling standards

Because requirements vary by jurisdiction, it is important to check early with your local building department. Some areas require stamped plans and multiple inspections. Others offer a more streamlined route for ADUs.

Common Types of ADU Permits

Depending on the project scope, you may need multiple approvals. Common examples include:

  • Building permit: Covers the main structural work and overall construction.
  • Electrical permit: Required for wiring, outlets, lighting, and appliances.
  • Plumbing permit: Needed if the unit includes a bathroom, kitchen, or wet bar.
  • Mechanical permit: Covers HVAC and ventilation systems.
  • Grading or drainage permit: May apply if the build affects grading, runoff, or site drainage.

Some municipalities combine permits into a single process, while others handle them separately. Working with a contractor familiar with local ADU requirements, or a company that offers permitting support, can reduce friction.

How to Apply for an ADU Permit

While the details change by city and county, the process often follows these steps:

  1. Review local requirements Contact your planning office or check the city website. Many areas provide ADU guides or checklists.
  2. Prepare and submit plans Most projects require detailed drawings aligned with local code. Plans may need an architect or engineer stamp. Prefab providers often supply permit-ready drawings.
  3. Pay fees Costs vary widely, from a few hundred dollars to several thousand.
  4. Wait for review and approval Timeline can range from weeks to months depending on local workload and review type.
  5. Schedule required inspections Inspections often occur at key stages such as foundation, rough utilities, and final sign-off.

Permits: A Key Step Toward a Smooth Build

Permits are not just paperwork. They are an important part of creating a legal, safe, and livable ADU. While the process can look intimidating at first, it is manageable with a clear plan and the right support.