When it comes to building an Accessory Dwelling Unit (ADU), understanding the costs involved is crucial. That’s where SnapADU comes in. As a company that focuses solely on designing and building ADUs, we have honed our expertise in estimating costs effectively. We understand the intricacies of the process, from the initial design to the final construction, and have gained valuable insights into all the extra costs that can come up. With this knowledge, we are well-equipped to provide accurate and transparent ADU cost estimates. In this article, we’ll explore the various factors that can affect ADU costs in California.

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 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 ($10K), permitting (~$10-14/sqft), typical sitework ($25K), and adding required solar ($10-20K). Also assume an average of $20-50K of costs for additional work like utility upgrades, which will be specific to your property requirements.

Cost To Build an 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 $325K $650 1BR <500 sqft
2BR/1BA 750 sqft $275K $365 $365K $485 2BR <750 sqft
2BR/2BA 1000 sqft $340K $340 $430K+ $430 2BR >750 sqft
3BR/2BA 1200 sqft $360K $300 $450K+ $375 3BR all sizes

Source: Snap ADU (updated May 2023)

How much it costs to build a detached ADU will be most heavily impacted by 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.

You’ve probably heard about the COVID-induced building price volatility. Build costs have gone up massively over the past two years, due to both material and labor shortages. The California Construction Cost Index (CCCI) went up 13.4% in 2021, another 9.3% in 2022, and is back near all-time highs as of May 2023… for a total increase of 30% from January 2021 to May 2023. That means an ADU built for about $300,000 in Jan 2021 would cost $384,000 to build in now in 2023.

As of this writing in May 2023, we have taken those costs into account for the figures above for ADU costs in San Diego. We want you to be prepared for what the real costs are going to be vs. leave you in for an unpleasant surprise later. You can read more about how we handle pricing at Snap ADU, including our price guarantee.


ADU Feasibility, Design & Site Assessment: $2,500

We “front load” a lot of the work that has historically been done later in the process. We want to assess the feasibility and all costs to build the ADU on your lot. The first step is to produce a site plan, map existing utilities, assess electrical panel loads, and create a feasibility report with your selected ADU plan mapped on your property. This will allow us to produce a guaranteed price for your project.

Construction Drawings: $7,500

Even with standard ADU plans, the ADU must be situated & adapted to your property specifics so that we can submit construction documents and application package with all required forms to the city. This comprehensive package includes  all required architectural design, structural engineering, title 24, building code review with the city, permit processing & any required revisions.

Additional Design: $2,500+

(OPTIONAL OR POSSIBLY REQUIRED) You can chose to modify any of our standard ADU floor plans to suit your needs. If you are modifying a floorplan, require a septic system or fire sprinklers there will be additional design time and costs incurred during the design phase.

Permits & Fees: $5,000+

While many ADU fees have been waived, there are still several due. A good rule of thumb is $10-12/sqft in ADU permit fees, though this varies by city and size. No impact fees apply for ADUs under 750 sqft; for ADUs >750sqft, impact fees are proportional to the size of the ADU in relation to the primary residence. School fees are also due, as well as sewer connection fees.

Basic Site Work: $25,000

For a level lot with sufficient access for construction equipment, typical sitework to prepare a buildable pad for the ADU will cost $25K. This includes trenching for the new utilities that will service the ADU. If you are rerouting utilities or building on a slope, additional costs can quickly escalate; read more below.

Additional Sitework: TBD

(OPTIONAL OR POSSIBLY REQUIRED) Additional ADU costs may come up based on your specific site. These are the biggest cost drivers we help you plan for. If you have a challenging property, you may also want to consider property reports (survey, title report, or a soils report) to properly plan for the ADU.

Vertical Building Construction: $200,000-400,000

The vertical building cost is for the ADU itself, including the foundation, structure and standard finish materials. Check out our floor plans for vertical build prices and our home features for a list of everything included in the build.

Upgraded Finish Materials: TBD

Standard finish materials are included in our vertical build prices on our plans page. Our basic finishes include electrical and plumbing fixtures, cabinets, and quartz countertops from our standard packages. You can also choose to upgrade any of these from our full design catalog.

Solar: $10,000-$20,000

All new construction detached ADUs are required to have solar, unless the main home already has solar installed, in which case that system may be expanded. Basic solar systems cost $15-20K to design & install depending on ADU size and energy use requirements (but you can claim a 30% federal tax credit on solar).

ADU 101 Webinar

Why is it so expensive 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:

  1. 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.
  2. 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.
  3. 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.
  4. Vendors: Wither 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.

What drives most of the cost of the ADU in California?

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 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. The first step is to let us know the address where you want to build and what size ADU you're considering... we'll start a proposal for you within 24 hours.

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ADU Build Costs by Floorplan

Click into ADU floor plans below for the cost to build each design, including standard finishes and appliances. We can put together a proposal for your specific property when you are ready to get into more cost details for your ADU.

Floor Plans

SnapADU 2BR/1BA 28×27 Square
2 Bed • 1 Bath • 749 ft2
SnapADU 3BR/2BA 36×31 L-Shape
3 Bed • 2 Bath • 999 ft2
2 Bed • 2 Bath • 1,199 ft2

ADU Sitework & 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.

Sitework Costs for your ADU

The Snap ADU 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 the first part of our design & feasibility process. That means you will have a locked ADU cost within 2-4 weeks of kicking off your project with us so that you can plan for the financials of getting your ADU built.

We go to great lengths during Feasibility & Design to make sure and uncover sitework roadblocks upfront. Over time, we have built out our own extensive ADU cost calculator so that we can give you a firm number early in the process. Because of this approach, you may end up seeing a higher cost in our proposal than any other bids you get. We want to give a realistic number for the all-in cost for your ADU so there are no surprises during the buildout.

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.