The Complete ADU Construction Guide: How To Build an ADU

Accessory Dwelling Units (ADUs) are secondary units that can be built on residential properties, providing an excellent opportunity for homeowners to boost property value, generate rental income, or house family members. 

However, understanding the many steps and rules involved in ADU construction in California can feel overwhelming. In this guide, we walk you through the key steps for building an ADU, from zoning and permitting to design and construction. Use these insights to understand how ADU building works and whether an ADU is worth the investment for you.

What Does ADU Stand For in Construction?

An ADU, or accessory dwelling unit, is a secondary dwelling built on a residential property with an existing primary home.

You might also hear people use incorrect terms like “additional dwelling unit,” “auxiliary dwelling unit” or “accessible dwelling unit,” but ADU construction always refers to a separate living space paired with a main home.

What are ADUs Used For?

An ADU can serve as a rental, guest home, workspace, or flexible living option. Other common names include granny flat, casita, guest house, companion unit, in-law suite, carriage house, and more. No matter the name, the concept of building an ADU remains the same.

Is ADU Construction Different Than Other Residential Construction?

Yes. A new construction ADU is unique because it combines complex state and local regulations with the challenges of adding a new structure to an existing property.

California ADU laws aim to streamline approval, but HOAs and local agencies may layer on additional rules. These overlapping regulations often create conflicting requirements, especially around design, placement, utilities, and setbacks.

Unlike new construction on open land, ADU building is an infill project, which means the structure must fit between existing features like homes, driveways, and landscaping.

Tight spaces, tricky grades, and utility constraints can create unexpected hurdles. The evolving regulatory environment and the infill nature of ADUs can lead to unexpected roadblocks, potentially causing delays ranging from days to months.

How Does the Cost Compare?

ADU costs also differ. ADU construction often has a higher cost per square foot because homeowners must pay for specialized planning, utility upgrades, and strict code compliance across a small footprint.

Who Can Build an ADU in California?

Any homeowner with an existing primary residence can build an ADU on their property, as long as they meet state and local rules. This applies to both single-family homes and multifamily properties.

While state laws have been designed to streamline the approval process and reduce barriers to ADU construction, local jurisdictions may have additional criteria or restrictions.

This is especially true in areas with unique geographical or environmental considerations, such as the Coastal Zone or historic districts. These regulations include required setbacks for the ADU on the property.

Considering a DIY approach? Read about our ADU shell option.

Benefits of Building an ADU

Building an ADU offers strong financial, lifestyle, and long-term benefits, including:

• Increased Property Value: ADU construction expands livable space and often raises resale value. They also create income-producing potential. Read about the ROI of an ADU.
• Rental Income: Many ADUs make reliable long-term rental income possible, which can help offset mortgage or living expenses. See our rental guide.
• Flexibility for Families: Many people build ADUs for extended family, aging parents, guests, or adult children. Read our family guide to ADUs.
• Affordable Housing Solution: ADUs add gentle density and help ease local housing shortages.
• Reduced Housing Costs: If you’re looking to downsize, living in your ADU while renting out the main home can be a smart financial move.
• Sustainability: ADU building uses land more efficiently and is often more environmentally friendly than large home additions. Read about sustainable design.

Learn more about the 8 key benefits of building a backyard home.

Requirements for Building an ADU in California

Building an ADU in California involves navigating state and local rules. California ADU law permits the construction of ADUs on residential lots that have an existing residential dwelling. State regulations set baseline standards for ADUs; local jurisdictions may impose stricter or more lenient criteria.

It’s crucial for homeowners to understand that while state laws aim to standardize ADU home construction across California, the specifics can vary significantly from one city or county to another.

Check your city’s legislation on ADU builds to ensure you are compliant on both a state and local level, or work with an expert like SnapADU who can check this for your ADU construction in Greater San Diego.

ADU Construction Types

Before diving into the specifics of building an ADU, it’s important to note that the rest of our guide focuses on stick-built construction, also known as wood framing.

This is a traditional home construction method where the structure is built on-site from the ground up. Stick-built ADUs offer unparalleled customization and flexibility, allowing homeowners to tailor every aspect of the design to fit their specific needs and preferences.

Stick-Build Vs Prefab & Modular ADUs

For those weighing the differences between stick-built, prefab, and modular ADUs, we recommend exploring our comprehensive guide on Prefab vs Stick Built vs Modular ADUs to understand the pros and cons of each construction method.

This guide will help you make an informed decision on which construction type best suits your project’s requirements, budget, and timeline.

Read about attached vs detached ADUs.

Step-By-Step Guide to Building an ADU

The ADU construction process involves several major steps, each with specific tasks, potential pitfalls, and local requirements.

1. Feasibility & Design

Once you decide you are interested in an ADU, an initial feasibility study is carried out to be sure this project can actually be completed… and on budget. This involves examining property setbacks, zoning, topography, easements and anything that might affect a successful ADU build. Your build may also require additional site work depending on its condition, so it is important to be sure you are clear on the anticipated budget to prepare your site for an ADU. If you are building on a small lot in a dense neighborhood, a survey may also be required.

ADU Placement on Your Property

While figuring out the best placement for your unit on the property, you will also finalize your design for the ADU. When determining the best floor plan for your lot, you must consider your personal budget, the amount of space available to you, and the intended use of your ADU. Will you want an extra room for hobbies, or a larger living space for entertaining?

Optimizing Your ADU Budget

You may also be working within a strict budget and looking to maximize your cost per square foot. These are things to take into account when determining your ADU floor plan. Your contractor will also be able to advise you towards the best layout for your unit.

Once your ADU design is set, and you have reviewed it to ensure it is to your liking, all of your specifications will be rolled into a full set of construction documents that will be submitted to your city.

2. Permitting & Revisions

The process of obtaining permits for your ADU project kicks off with the assembly of a set of construction documents (CDs) that outline all the details of your ADU- even down to the location of each light switch.

The ADU Permitting Process

Your city will review the CDs to make sure the proposed build will be carried out in accordance with their regulations and laws. It is most common for the city to return these plans with a few revisions that must be made before the permit can be issued.

Not to worry though, your contractor will have planned for these revisions and will be prepared to make them. During this time, you will also be making your finish material selections so that the items can be ordered in time for the build.

ADU Permitting Fees

Throughout the permitting process, you will be required to pay various plan check and permitting fees depending on the size of your unit. Some jurisdictions allow certain fees to be waived if your unit lies under a size threshold.

Once these fees are paid and revisions are made, you will be ready to begin mobilization and building. At that point, your permit is officially issued and the construction timeline begins. You can read more about how long ADU permits last in San Diego and the deadlines to start construction.

3. Mobilization

Once an ADU construction project is permitted, construction mobilization begins within two weeks. The ADU’s footprint is located on the property with stakes to visualize the unit in relation to existing features of the property, and to get an idea of any necessary site work that will need to be taken care of before the build.

How the Mobilization Phase Works

This may include removing foliage and rocks, or filling in a hole. You may also see the temporary toilet facility moved onto the job site during the mobilization phase.

Your project manager will assemble the approved documents and bring them on-site for reference during the ADU build, where you will meet and discuss the next few steps of the project. Your superintendents will continue to oversee work on-site throughout the job.

4. Site Work

The groundwork for ADU building begins with site preparation and grading, ensuring a stable base for construction. This step often involves:

• Clearing the Site: Removing vegetation, rocks, or debris that could interfere with construction.
• Grading and Compaction: Leveling the soil to prepare for the foundation, with added work for sloped properties or installing retaining walls.
• Utility Preparation: Running essential lines for electric, water and sewer, as well as potential electrical panel upgrades or adding a septic system.

This is a critical phase in the ADU construction process where unexpected challenges, like poor soil conditions, can arise. Working with a skilled contractor ensures these hurdles are addressed efficiently.

5. Trenching & Foundation

This stage lays the structural base of your ADU. Key tasks include:

• Trenching: Excavating around the ADU’s footprint to accommodate utilities and foundation forms.
• Formwork & Reinforcement: Installing wooden formwork and reinforcing steel bars for stability.
• Concrete Pouring: Filling forms with concrete, ensuring a level surface for construction.

Proper execution here impacts the durability and longevity of your unit. Choosing the right foundation type depends on site conditions and local codes.

6. Framing

Framing is the part of the ADU construction process that brings your ADU structure to life. This involves:

• Wall & Roof Framework: Erecting walls, ceilings, and roof frames with precision.
• Openings for Doors and Windows: Ensuring accurate placement of openings for proper installation later.
• Sheathing: Adding structural panels to walls and roofs for strength and weatherproofing.

ADU Framing progresses rapidly and also requires expert alignment to meet building codes and design specifications.

7. Rough MEP (Mechanical, Electrical, Plumbing)

With the structure intact, critical systems are integrated:

• Plumbing Rough-In: Installing water supply and drainage lines.
• Electrical Setup: Routing wires, installing electrical boxes, and ensuring compatibility with fixtures.
• HVAC Installation: Installing mini split units or forced air for heating and cooling.

This step ensures the ADU is equipped with essential utilities before walls are sealed.

8. Exterior Finishes / ”Water Tight”

In this phase of the ADU construction process, the exterior is sealed to protect against weather while work on the interior begins:

• Roofing: Installing shingles or other materials to waterproof the structure.
• Windows & Doors: Fitting door and window elements for insulation and security.
• Siding or Stucco: Adding the chosen exterior finish per plan, typically to match the main dwelling.

Achieving a watertight exterior allows for simultaneous interior work and exterior detail refinement. This phase protects the structure from rain and moisture, which is a normal and expected part of the ADU construction process.

Many homeowners worry about weather exposure during framing, but modern building methods are designed to handle this safely. You can learn more about why rain isn’t an issue for ADU construction and how builders protect your structure at every stage. Read more about exterior finishes.

9. Insulation & Drywall

Insulation & drywall prepares the ADU for comfort and aesthetics:

• Insulation Installation: Filling walls and ceilings with insulation to regulate temperature and meet energy requirements.
• Drywall Installation: Hanging, taping, and finishing drywall to create smooth surfaces.

It’s at this stage that some homeowners choose to take over the ADU project themselves. Read more about our DIY-friendly “tape and texture” ADU option, where we complete the project up until drywall so you can handle finish work.

10. Interior Finishes

The finish selections you made during the permitting phase finally come to life in this stage of ADU building:

• Painting & Flooring: Applying paint and installing flooring materials.
• Cabinetry & Countertops: Fitting cabinets and counters in kitchens and bathrooms.
• Lighting & Fixtures: Installing light fixtures, faucets, and other final details.

These touches personalize the ADU and align it with your vision. See examples of completed ADU kitchens and ADU bathrooms.

11. Final Touches for Building an ADU

The finishing phase ensures your ADU is move-in ready:

• Cleaning & Detailing: Removing construction debris and perfecting surfaces.
• City Inspections: Passing final inspections to secure a Certificate of Occupancy.
• Landscaping: Enhancing the outdoor space to complement the new unit.

This is the moment where your investment comes to life, ready for its intended use. Many homeowners will complete landscaping work around their ADU as a second phase of the project. There are many landscaping ideas you can implement – for inspiration check out outdoor landscaping products.

Read more: How to Prepare for an ADU Build: 7 Steps To Take Before Starting

Factors Contributing to ADU Construction Costs

Building an ADU involves many cost factors, including design, permitting, site preparation, utility upgrades, and of course on-site construction. Factors such as the size of the unit, complexity of the design, local building codes, and utility connections can all impact the final price. Then there are geographic considerations, like lot conditions and city-specific regulations, may require extra site work, increasing overall expenses.

ADU Cost Saving Strategies

Luckily, there are cost-saving strategies that homeowners can explore, such as opting for pre-approved designs, maximizing existing space, or building multiple units simultaneously to take advantage of economies of scale. Understanding these factors can help you plan your project more efficiently and keep costs within budget.

Read more about ADU cost saving strategies.

Core Costs for Every ADU Build

Regardless of the scope or size of your project, several core costs apply to every ADU build. These essential expenses ensure the project progresses smoothly from feasibility through to construction.

• ADU Feasibility Study. A comprehensive feasibility study that covers design, surveys, utility mapping, and a full analysis tailored to your property and ADU floor plan. This study is essential for an accurate assessment of your project’s scope and costs.
• Construction Drawings. Architectural design, structural engineering, Title 24 compliance, and all necessary revisions to meet city standards for permitting.
• Permitting Management: Entire application and approval process, from form submission to city plan checks.
• Vertical Building Construction: The primary cost of the ADU construction, including foundation work, structure, standard finish materials, and warranty. Costs vary depending on size and design complexity.
• Basic Site Work & General Conditions: Includes necessary site preparation like trenching, utility connections, and finish grading on a level lot with accessible construction equipment. General conditions also includes dumpsters, temporary fencing & toilets, and signage.

 

Additional Costs for ADU Builds

In addition to the core ADU construction costs, there are property-specific and city-specific expenses that can vary depending on your unique project needs and local regulations.

Property-Specific Costs

These costs depend on the characteristics of your lot and any special requirements based on its condition.

• Additional Sitework: Additional site preparation may be required, such as demolition of existing structures, grading, retaining walls, or stormwater mitigation, which can significantly impact your overall costs.
• Additional Utilities: Most ADUs require separate utility meters or other electrical upgrades. Solar requirements may apply, and some properties might need specialized sewer and water work or septic system installation.
• Additional Vertical Building Costs: Unique property features, such as the need for special foundations or fire-rated construction, can increase vertical build costs. Additionally, choosing custom exterior finishes or complying with HOA requirements will also add to the price.

 

City-Specific Costs

These additional expenses are determined by local regulations, permitting fees, and city-specific building codes.

• Permits & Fees: Permit fees vary by city but typically range around $10-15 per square foot. Some impact fees, school fees, and sewer connection fees may also apply, particularly for larger ADUs.
• Property Reports: Some jurisdictions require specific reports like boundary surveys, grading plans, or soils reports. These are needed to comply with local regulations and can add to the overall project cost.

 

By understanding both core and additional costs, you can better plan for your ADU project and identify opportunities to save on expenses. Explore more on ADU costs.

Contractor Selection and Management

Choosing the right contractor is one of the most crucial steps in your ADU construction project. From selecting a contractor with the right experience to managing the construction process effectively, each decision can significantly impact the success of your build.

As a leading ADU construction specialist in Greater San Diego, we’re happy to work with you on a local project, or read on to learn what to look for in a contractor elsewhere.

The Importance of Selecting an Experienced ADU Contractor

Selecting an experienced ADU contractor ensures your project stays on track and meets all regulatory requirements. Contractors specializing in ADU builds are familiar with the unique challenges of these projects, including tight space constraints, local zoning rules, and utility integration.

Remodeling is different from building new construction ADUs.  Learn more about how to find the right ADU builder for your project, ensuring they have the relevant experience, proper licensing, and insurance coverage. Also review the considerations of working with “one contractor that does everything.”

How to Compare ADU Builder Quotes

When comparing contractor quotes, make sure you’re assessing “apples to apples” by looking at the full scope of work, including site prep, utility work, and finishes.

Different builders may provide varied levels of detail in their estimates, so clarity is essential to avoid surprises down the line.

For guidance on how to make informed decisions, check out our article on comparing ADU builder quotes and what to watch for in terms of scope, costs, and exclusions.

Also consider ADU Contractor Red Flags: Warning Signs to Watch Out For. We include the subtle phrases that signal an ADU construction company may not respect your time, budget, or goals…. and what to ask to find legitimate contractors.

Understanding the Design-Build Approach for ADU Construction

Working with a design-build firm streamlines the process by combining design, permitting, and construction under one contract.

This eliminates the need for separate architects and contractors, helping ensure clear communication and accountability. The design-build approach simplifies contractor management and reduces potential delays or miscommunication between design and construction phases.

The Advantage of Working with Experienced ADU Construction Companies

SnapADU has literally hundreds of planned schedule items and to-do lists for our team and our subcontractors so that each project runs smoothly. We have built out these templates over years of designing, permitting and building ADUs.

Unique to a design build contractor is the ability to make changes to the design & permitting phase that will make the build-out easier and more predictable. And we are also prepared to quickly recover when needed. Our blog on the 7 habits of top ADU builders will give you a checklist of things to look out for.

At SnapADU, we know exactly what the ADU-specific inspection requirements will be, which means we can incorporate those items into the plans to prevent downstream delays. We pride ourselves on managing every aspect of the ADU construction process with precision and care, ensuring a successful build.

Work With a Snap ADU Specialist on Your ADU Build

Let us know a bit about your ADU project so we can provide feedback on feasibility, cost and timeline for your unique accessory dwelling unit. You can also read more about turning your ADU planning process into action.

VIDEO: What to Expect When Building an ADU