Accessory Dwelling Units (ADUs) are secondary units that can be built on residential properties, providing an excellent opportunity for homeowners to boost property value and generate rental income. However, understanding the many steps and requirements in California for ADU construction can be challenging. In this blog, we will provide you with step-by-step guidance on how to successfully build an ADU in California, from understanding the zoning laws and permitting process, to designing and constructing your guest house. Get ready to unlock the potential of your property with our expert tips and insights!
What does ADU stand for in construction?
The term ADU stands for “accessory dwelling unit” and refers to a secondary dwelling unit on a residential property that exists in addition to the primary dwelling or residence. You might also hear ADUs mistakenly referred to as an “additional dwelling unit,” “auxiliary dwelling unit” or “accessible dwelling unit.”
To be considered an ADU, there must already be a primary dwelling unit on the property. Accessory dwelling units are incredibly flexible spaces and can be used as a rental, a guest home, or even a personal workspace. Other terms for ADU include granny flat, companion unit, casita, guest house, granny cottage, secondary living unit, mother-in-law apartment, second dwelling unit, in law suite, carriage house, au-pair suite… you may hear any of these terms used interchangeably for an accessory dwelling unit, but the meaning is all the same.
Is ADU construction different than other residential construction?
Yes, accessory dwelling unit construction is distinct from other residential projects due to unique regulatory challenges and the nature of the work involved.
ADUs face a complex blend of state and local regulations, often leading to intersecting and conflicting regulations. While state ADU laws in California aim for uniformity, local entities and Homeowners Associations (HOAs) add another layer of rules. Unaddressed issues in state laws leave room for varied local interpretations and solutions.
Detached ADUs also differ, as they are “infill” projects, not new construction on empty land or simple renovations. Adding an ADU to a home requires integrating new structures into existing spaces, often with limited room due to specific site constraints and setbacks.
This evolving regulatory environment and the infill nature of ADUs can lead to unexpected roadblocks, potentially causing delays ranging from days to months. The resolution of these issues and the associated costs can vary significantly. Also, ADU costs are often much higher than homeowners expect. The cost per square foot for ADUs is often higher than for larger new homes, as the expenses for specialized planning, utility upgrades, and strict code compliance are spread over a smaller area, increasing the overall cost intensity of the project.
Who can build an ADU?
In California, any homeowner who has an existing primary residence on their property is eligible to build an accessory dwelling unit (ADU), provided they comply with both state and local laws governing ADU construction. This opportunity extends to owners of single-family homes as well as 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, especially 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 more DIY approach? Read about our ADU shell option.
What are the benefits of building an ADU?
Building an Accessory Dwelling Unit (ADU) offers numerous benefits for homeowners, whether you’re looking to increase property value, generate rental income, or create additional living space. Here are some of the top advantages:
- Increased Property Value: ADUs can significantly boost your property’s resale value by adding additional living space and potential rental income streams. Read about the ROI of an ADU.
- Rental Income: Many homeowners use ADUs as rental units, providing a steady income that can help cover mortgage payments or other expenses. See our rental guide.
- Flexibility for Families: ADUs offer convenient housing for extended family members, such as aging parents, college students, or guests, allowing for independent living with close proximity. Read our family guide to ADUs.
- Affordable Housing Solution: By building an ADU, you contribute to addressing the housing crisis by providing affordable living options in your community.
- 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: Building an ADU is often more environmentally friendly than larger construction projects since it maximizes existing land use. Read about sustainable design.
Learn more about the 8 key benefits of building a backyard home.
What are the requirements for building an ADU in California?
Building an accessory dwelling unit in California involves navigating a series of state and local regulations designed to ensure these structures meet specific standards for safety and design. California law permits the construction of ADUs on residential lots that have an existing dwelling. State regulations cap the size of detached ADUs at 1,200 square feet and limit their height to 16 feet. However, these are baseline standards; local jurisdictions may impose stricter or more lenient criteria. It’s crucial for homeowners to understand that while state laws aim to standardize ADU construction across California, the specifics can vary significantly from one city or county to another.
So, it is of the utmost importance to check your city’s legislation on ADU builds to ensure you are compliant on both a state and local level.
What ADU construction types are there?
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, a traditional 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. 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 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.
SnapADU Team During Construction
What are the steps to building an ADU?
The process to build a granny flat from scratch is carried out in roughly ten steps. Within each of these steps, there are numerous tasks to complete, pitfalls to be aware of, and regulations within your jurisdiction that must be considered.
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.
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?
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 that outline all the details of your ADU- even down to the location of each light switch. Your city will review these documents 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.
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.
3. Mobilization
Once an ADU project is permitted, construction mobilization begins within 2 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. 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 construction, 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 your ADU 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 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 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.
Framing progresses rapidly but 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”
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. 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 a build:
- 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.
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. Additionally, geographic considerations, like lot conditions and city-specific regulations, may require extra site work, increasing overall expenses. However, 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.
Core Costs for Every ADU Build
No matter the scope or size of your ADU project, several core costs apply to every 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.
Additional Costs for ADU Builds
In addition to the core 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.
How To Address Rain During ADU Construction
Incorporating rain considerations into your ADU construction project is essential to ensure durability and compliance with local regulations. While San Diego’s climate is predominantly dry, unexpected rain events can occur, and it’s crucial to be prepared.
For example, while framing lumber may be exposed to rain during the initial phases of construction, most materials used are designed to tolerate short-term moisture without compromising their integrity. Shearing the entire structure with plywood – although not always required – is a common best practice that provides additional stability and helps the building dry out more quickly. The plywood used is typically moisture-rated, which adds an extra layer of protection.
Mechanical systems, electronics, and other sensitive components are prioritized for protection during rainy periods. Once interior finishes like drywall are ready to be installed, the roof is typically completed to avoid moisture intrusion. Inspections are also scheduled to align with these critical milestones to ensure everything is properly sealed before moving on to interior work.
While rain may introduce minor delays, thoughtful scheduling and contingency planning help construction projects adapt without significant impact to the timeline or quality.
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. Here’s how to navigate contractor selection and management for your ADU project.
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. Learn more about how to find the right ADU builder for your project, ensuring they have the relevant experience, proper licensing, and insurance coverage.
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.
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. For instance, 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. At SnapADU, we pride ourselves on managing every aspect of the ADU project with precision and care, ensuring a successful 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.
VIDEO: What to Expect When Building an ADU
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