ACCESSORY DWELLING UNIT COSTS
We understand ADU costs in California.
When it comes to building an Accessory Dwelling Unit (ADU), understanding all the costs is crucial.
At SnapADU, we’re about transparency and clarity, especially when it comes to the costs involved in building an ADU. We don’t just design and permit ADUs, we also build them, giving us firsthand insights into every hidden cost that can spring up along the way.
The process of building an ADU is layered, from initial planning to the final stages of construction. Our experience in ADU design build has taught us how to anticipate and manage unforeseen costs that often slip past the design and permitting phase. It’s our responsibility to foresee these potential pitfalls, armed with a wealth of experience and knowledge about real-world ADU costs.
In this article, we’ll delve into the comprehensive costs of building an ADU, discussing all the variables that can influence the final price in the San Diego, California, area. We’re here to give you a realistic view of what to expect financially when embarking on your ADU project.
How much does it cost to build an ADU in San Diego, California?
For budgeting purposes, figure on the ADU cost per square foot running $375-600+ per square foot for your all-in cost to build a turn-key detached ADU, for units ranging from 500-1200 sqft. Roughly 80-85% of those costs are for the vertical building construction (the construction of the structure itself, e.g. the building and standard finishes).
To get to the all-in cost to build a granny flat, also factor in the ADU costs from design ($12K), permitting (~$2-14/sqft in fees depending on city), and typical sitework ($25K). Also assume an average of $30-50K+ of costs for additional work like utility upgrades, which will be specific to your property requirements. We review over 75 factors for each property to assess all the relevant costs for your ADU project.
TABLE: Cost To Build New Construction Detached ADU in San Diego, California
| Type | Size | Vertical Build Cost* | Build cost per sqft | Typical All-In Cost | All-in cost per sqft | View Example Plans & Prices |
|---|---|---|---|---|---|---|
| 1BR/1BA | 500 sqft | $235K | $475 | $300K | $600 | 1BR <500 sqft |
| 2BR/1BA | 750 sqft | $275K | $365 | $350K | $465 | 2BR <750sqft |
| 2BR/2BA | 1000 sqft | $340K | $340 | $425K | $425 | 2BR >750 sqft |
| 3BR/2BA | 1200 sqft | $360K | $300 | $450K | $375 | 3BR all sizes |
Source: Snap ADU (updated January 2025). *Vertical Build Cost includes finishes
How much it costs to build a detached ADU will be most heavily impacted by the size unit, how many units you are building, site conditions (e.g. grading, utility locations) and local requirements for permitting (e.g. building plan check & permitting fees, school fees). Vertical build costs will be affected by final finishes; our pricing always includes a standard level of finish, including appliances and fixtures. See more about our standard features.
It’s crucial to acknowledge the recent surge in build costs, which can often lead to sticker shock for many homeowners. The California Construction Cost Index (CCCI) has increased a total of 37% from January 2021 to January 2024. This hike means that an ADU which cost around $300,000 to build in January 2021 would now require an investment of approximately $410,000 in 2024.
As of June 2024, our cost estimates for ADUs in San Diego have been adjusted to reflect these changes. Our goal is to provide you with a realistic understanding of the expenses involved, rather than leaving you with unexpected costs later. You can find more detailed information on our pricing approach and our price guarantee. Being well-informed from the start can help you plan effectively for your ADU project.
How much does it cost to build multiple ADUs?
The cost of building multiple ADUs varies widely depending on factors like size, design complexity, site constraints and utility upgrades. Generally, you can expect some cost savings through shared components, such as a single foundation and exterior finishes. However, each unit will still require its own kitchen, bathroom, HVAC, and electrical systems, which are significant cost drivers.
As a rough estimate, plan for the second unit to cost about 85% of the first unit. Overall, while there are efficiencies in mobilization and project management, the cost per square foot will not decrease as much as simply adding square footage to a single ADU. For a detailed cost assessment, consulting with ADU construction companies is essential.
Read more about stacked ADU costs and considerations.
What is the total cost to build an ADU?
ADU Feasibility Study: $7,500
Our comprehensive ADU Feasibility Study goes far beyond basic reports that others offer (we cover that level of information for free in our Discovery Process & Proposal). Your in-depth study will include an accurate assessment of the scope & costs for your ADU project. It incorporates essential elements such as surveys, utility mapping & planning, design, renderings, and a complete analysis and budget tailored to your property and floor plan.
Construction Drawings: $7,500+
Our in-house drafting team produces a full set of ADU plans required by the city, known as construction documents (CDs). This comprehensive package includes architectural design, structural engineering, Title 24 compliance, building code reviews, and necessary revisions to ensure your ADU meets all standards for permitting. Standard one-story plans start at $7,500, with additional costs for multi-story units or complex designs.
Additional Design: $2,500+
(Applies in some situations) Our in-house design team can support you with custom design work if you need to modify one of our standard ADU floor plans or design a completely custom plan. This service is ideal if you’re adding a garage, building multiple ADUs, or dealing with special foundation scenarios. Custom design work typically extends the design timeline by 2-6 weeks, depending on the complexity of the changes.
Property Reports: TBD
(Applies in some situations) Depending on your jurisdiction, as well as where you intend to locate the ADU on your property, additional reports may be required to properly plan for the ADU. These could include a boundary survey, title report, soils report, and grading plans. We’ll determine the need for these reports during our discovery process with you and incorporate them into your project proposal.
Permitting Management: $2,500+
Each ADU project requires an application package with all required forms for plan check. Effective advocacy plays a crucial role in navigating the permitting process. Our in-house permitting team handles these steps from start to finish, ensuring a seamless and efficient experience. Additional permitting management costs could apply if you are in the coastal zone, an HOA, or in jurisdictions requiring in-person submittals or other special requirements.
Permits & Fees: $5,000+
While many ADU fees have been waived, several are still applicable. A good rule of thumb is to budget around $10/sqft in ADU permit fees, though this varies greatly by city and unit size. For ADUs over 750 square feet, impact fees proportional to the ADU’s size relative to the main house may apply. School fees and sewer connection fees are also part of the overall cost.
Basic Site Work & General Conditions: $35,000
For a level lot with adequate access for construction equipment, basic site work is needed to prepare a buildable pad for the ADU. This includes trenching & connecting utilities, finish grading, and general conditions like required site protection, temporary facilities, and necessary testing for soils compaction and energy efficiency.
Vertical Building Construction: $200,000-400,000+
The vertical building cost is for the accessory dwelling unit itself, including the slab-on-grade poured foundation, structure, standard finish materials, and warranty. View our floor plans for vertical build construction prices and our home features for a list of everything included in the build.
Finish Material Upgrades: TBD
(Optional) Standard finish materials are included in our vertical build prices. Wood cabinets, quartz countertops, appliances, flooring and fixtures are all included in our standard packages. You can also choose to upgrade any of these selections from our full design catalog, which includes tiled showers, islands, and premium appliances.
Additional Sitework: TBD
(Applies in some situations) Additional sitework costs may come up based on your specific property features or jurisdictional requirements. These are the sitework cost drivers we help you plan for, including demolition of existing flatwork, structures & trees, grading, retaining walls, and stormwater mitigation (required in some cities). When required, soils reports also necessitate additional sitework.
Additional Utilities: TBD
(Applies in some situations) Most properties require a separate electric meter for the ADU, and other special electrical requirements may apply. Some new detached ADUs are required to have solar, though many are exempt based on size and climate zone. We’ll also look for any additional sewer and water requirements, or plan for septic if needed. Typically we see all these costs ending up between $15-30K+ depending on property specifics.
Additional Vertical Building: TBD
(Applies in some situations) Depending on your property & zoning specifics, you may require a special foundation type or fire rated construction, both of which increase the cost to build the unit. Choosing non-standard exterior finishes – like siding or tiled roofing – will also add some cost. HOAs may also mandate some exterior upgrades so that the ADU matches your main home.
What drives most of the cost of the ADU?
The construction itself is by far the most expensive part of adding an ADU. As you can see in this example, sitework & the structure make up 94% of the overall project costs; plans & permits are just 6%. This means your contractor is a vital part of the project, since they will drive the vast majority of the costs (and hopefully not incur overages). Additionally, we suggest design/build as a more predictable model, especially since we provide a fixed price for the build upfront.
When can we talk about my ADU costs?
We want to talk with you about the cost to build an ADU from Day 1 because we think it’s vital to align design with your budget constraints. With over a decade in construction, we have fine-tuned our process for delivering accurate & realistic construction estimates that we stand behind with a price guarantee. We want to give you a picture of the full costs of the granny flat project, from design & permitting, to site work & construction.
Whether you are just starting to think about adding an ADU or are ready to go, we can give you a cost to build an ADU so you have the information you need to move forward with your project.
ADU Site Work & Utilities: 10 Sneaky Costs to Watch Out For
It’s easy to tell you how much the ADU structure itself (also known as the “vertical build”) will cost. But the most variable part of the cost to build an ADU is the sitework. The exact same unit could cost tens of thousands more to build on a challenging property than it costs to build on a flat lot with good access. Check out our guide to sitework costs so you know what to look out for when budgeting for your ADU costs.
Why does it cost so much to build an ADU?
We often are asked why Accessory Dwelling Unit building costs seem higher when compared to a Single Family Residence (SFR) on a cost per square foot basis. In short, the answer is that SFRs are often twice the size of an ADU, resulting in a lower price per square foot. ADUs have a higher cost per square foot because they bear many of the same “fixed” costs of any residential building project.
Whether building an ADU or a SFR, there are certain base costs which occur no matter the size of the structure. These include:
- Design & Construction Documents: Relatively speaking, drafting hours and costs are in the same ballpark for an ADU or a basic SFR. An ADU still requires a full set of building plans… even pre-approved plans are not something you can walk into a building department as-is; they must be customized to your property.
- General Conditions: Dumpsters, temporary toilets, project supervision, and site protection are required regardless of the build size and will be part of your ADU costs.
- Kitchen, Bath & Laundry: Kitchens are the most expensive room of a building, including cabinets, countertops, plumbing & sink, plus appliances. Bathrooms and laundry facilities are also costly with plumbing & finish materials. A small one bedroom ADU still needs a kitchen and a bathroom, while a modest SFR might need just an additional bath yet support 4X the square footage.
- Vendors: Whether a vendor is installing a small amount of material for an ADU or a large amount for a SFR, there is a base cost for simply showing up and setting up their tools.
Thus, there is a minimum base cost to build a residential unit, no matter how small the unit may be. That means that an ADU cost per square foot can be as high as $500+ for very small units.
Read more about how to compare ADU quotes. Looking to go a more DIY route to save money? Consider an unfinished shell ADU, where you complete the finish work yourself and save.
Is an ADU a good investment?
So now that you know about ADU cost in California, you may be wondering: is an accessory dwelling unit a good investment? Check out our article to think through how ADU cost, resale value, and other factors influence the answer for your specific property.
How will the LA fire affect construction costs in Southern California?
It’s been devastating to watch the Palisades and Eaton fires in Los Angeles County. These fires strike particularly close to home for us, as builders in this region and industry. While it is difficult to predict impacts of natural disasters like this, we can look to the past to make some educated guesses about what could play out.
What we know from past events of this scale
This is our initial research, and more details will emerge in the coming months. As the situation evolves, we’ll continue to monitor the ripple effects in our area.
Rebuild Efforts Take Years
Five years after the Camp Fire in Northern California that burned nearly 19K structure… just 21% of the homes lost had been rebuilt. Because of permitting & construction complexities, it’s unlikely we’ll see a significant portion of the LA homes rebuilt simultaneously.
Material Prices Often Stabilize Quickly
After the 2018 Camp Fire in Northern California, there was a short-term spike in lumber prices. However, these costs stabilized as rebuilding efforts got underway. Current reports suggest that North America’s lumber market is approaching equilibrium heading into 2025, meaning any cost increases may be more measured than in past years.
Subcontractor Relationships Matter
In times of increased demand, long-standing relationships with local subcontractors and suppliers help mitigate challenges. Contractors with established ties often have better access to labor and materials.
How Snap ADU Approaches Pricing and Cost Increases
While we can’t control broader market factors, we’re always working to minimize unnecessary costs and advocate for our clients.
Advocacy for Clients
We actively push back against unnecessary costs, such as excessive requirements from city inspectors or code changes, and prioritize micro deadlines that can mitigate broader impacts (e.g., submitting before costly code changes take effect, whenever possible).
Long-Term Relationships
We work with a consistent core group of subcontractors that are local to San Diego. For many, we are a significant portion of their business. This consistency helps with reliability, increasing our odds of navigating market challenges collaboratively.
Our Price Lock Policy
Your construction agreement provides clarity on your build costs. Once signed, SnapADU assumes liability for increases in construction labor and materials during the permitting process for up to 6 months. Any adjustments beyond that timeframe would align with the California Construction Cost Index (CCCI) and are capped to ensure fairness.
How will tariffs and rising costs affect my ADU project?
The cost of building an ADU is influenced by many factors, including labor availability, material costs, and overall economic conditions. While tariffs on building materials can contribute to price increases, their actual impact is often smaller than you might at first imagine. The bigger challenge is unpredictability, as it makes it difficult for homeowners to plan financing.
While tariffs can raise prices on certain imported materials, not all materials are affected. Also, a significant portion of an ADU’s cost is labor. For example, lumber is often impacted by tariffs, but when you break it down:
- Framing accounts for about 20% of an ADU’s total cost, so on a $400K build, that’s roughly $80K.
- Of that, 65-75% is the actual lumber cost—around $56K in materials.
- According to the NAHB, the US imports roughly 30% of the softwood lumber supply for the nation… so assume $16,800 is imports
- About 72% of U.S. lumber comes from Canada, so roughly $12K would be affected by tariffs
- If that Canadian lumber saw a 25% tariff increase, it would mean about $3K of additional costs – so about 0.75% of a $400K build.
This shows that while tariffs can impact material costs, they rarely cause drastic price jumps across an entire project. And as of today, despite ongoing tariff discussions, we have seen absolutely no impact on our pricing.
How SnapADU Manages Costs & Market Shifts
We have long-standing relationships with our trade partners and have established a reliable pipeline of 30-40 ADU projects per year, which helps us maintain stable pricing, even in fluctuating markets. A prime example is how we navigated the COVID-era cost surges from 2020-2022, when construction cost inflation (CCCI) spiked to 13.4%, 9.3%, and 9.4% in consecutive years.
During that time, we did our best to absorb or share cost increases where feasible to keep projects moving. While some projects ran at very limited margins, we prioritized client service and long-term business stability, ensuring that we could continue delivering high-quality ADUs. We have also never increased prices mid-build, largely due to the strong commitment from our subcontractors, who value our steady project pipeline and long-term relationships.
What If Costs Spike Mid-Build?
We know homeowners need to plan for worst-case scenarios. If costs were to rise dramatically midstream, there are natural break points in construction where we could pause and reassess. For example, we could take the project through drywall completion, at which point you would have a fully enclosed, insulated, and weatherproofed structure. From there, you could choose to:
- Modify finish selections to lower-cost alternatives.
- Handle some finish work yourself or through another contractor.
- Pause temporarily before final completion if needed.
While costs will continue to rise, waiting likely increases the financial challenge of building an ADU. At SnapADU, we’re committed to transparent pricing and a streamlined design-build process to ensure you get the most value out of your investment, even in a fluctuating market.
The SnapADU Price Lock.
Even though California has eased the process, ADU design & permitting still takes on average 4-6 months (and sometimes much longer if a city is backed up). During that time, the cost of the ADU build is typically in flux, as most builders won’t give a final cost until closer to breaking ground. If you’re financing the ADU, this can be a disaster, as your loan value might not cover the final build cost.
To offer homeowners more certainty in their ADU budget, we offer a Price Lock as part of our feasibility & design process. This guarantee ensures that your ADU cost is fixed by the time we submit the plans for city permits, enabling you to confidently plan your finances for the ADU construction.
We go to great lengths during Feasibility & Design to make sure and uncover sitework roadblocks upfront. Our extensive experience in accessory dwelling units has enabled us to develop a sophisticated ADU cost calculator, providing you with an accurate and realistic project estimate early on. It’s important to note that our initial proposal might appear higher compared to other bids not anticipating these requirements. This is because we aim to present a comprehensive and realistic cost, encompassing all potential expenses we are aware of, to avoid surprise costs during construction.