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San Diego's Pre-Approved ADU Plans - Should You Use Pre-Approved ADU Plans for Your Home?

Explore the benefits and drawbacks of using pre-approved ADU plans for your San Diego home and how Customizable ADUs can suit your needs in this guide.

February 6, 2023

Homeowners are investing in accessory dwelling units (ADUs) more than ever before. One way to simplify the process of building an ADU is by using pre-approved plans. The County of San Diego has made six cost-efficient county-owned designs with the opportunity for quicker implementation. But are pre-approved plans right for you, and are they designed to last?

Keep reading to find out what pre-approved plans are available, and frequently asked questions on this topic that would help you decide what works best for you.

A Quick Look at County-Owned Pre-Approved ADU Plans

A pre-approved, or permit-ready, ADU plan is a design that has been reviewed and approved by building authorities. County-owned plans are created by county and government architects and made publicly available at no cost. However, if you choose to go with a county-owned plan, you would need to handle the rest of the permitting process yourself, and hire external designers, architects and drafters to make structural calculations or any modifications to the site plan.

Still, pre-approved plans undergo fewer changes than custom plans, so it could take less time to obtain a permit for them. Here are the pre-approved plans currently available.

  1. Plan A: 1200 sq.ft., 3 bedrooms and 1 bathroom
  2. Plan B: 1200 sq.ft., 2 bedrooms and 1 bathroom
  3. Plan C: 1200 sq.ft., 2 bedrooms and 2 bathrooms
  4. Plan D: 1000 sq.ft., 1 bedroom and 1.5 bathrooms
  5. Plan E: 800 sq.ft., 1 bedroom and 1 bathroom
  6. Plan F: 600 sq.ft., 1 bedroom and 1 bathroom

The demand for ADU construction has risen steadily over the past few years as a result of the need for affordable housing, the ability to use ADUs for rental income and as space for extended families. As the demand for ADUs has grown, some cities have introduced pre-approved ADU plans to make the permitting process easier. But despite their availability, there are a few factors that you should consider before deciding if you want to use pre-approved plans for building an ADU.

Can Pre-Approved Plans be Submitted to the City Planner as-is?

Unfortunately not. Pre-approved plans don’t take into account the possible limitations of different building sites. You’d still need to draw up a site plan that details all the features of your property, including electrical services, proposed structures, gates, and drainage systems, to name a few.

Do You Need Help From an Architect if You’re Using Pre-Approved ADU Plans?

Technically, no, you don’t. But you would need to develop the site plan, make Title 24 calculations to report on your building’s energy compliance, conduct a property survey, and perform any required structural calculations. All of this would be hard to carry out without the support of an expert and your permits are likely to be approved quicker if a professional submits them on your behalf.

What Changes Can You Make to a Pre-Approved ADU Plan?

Pre-approved plans come with design constraints and limited room for personalization. While you cannot add square footage for a space that wasn’t originally included in the plan, some cities allow you to make minor edits like:

  • changing the size or placement of doors and windows
  • changing exterior materials
  • 25% of non-structural changes to the existing design

These allowances vary across jurisdictions but note that any change to the plan, no matter how small, must be made by the original architect and could mean losing its pre-approved status.

Are Pre-Approved ADU Plans Actually Faster to Implement?

This depends. It’s very unlikely that a pre-approved plan will be good to go as is, and while the plans themselves can be accessed for free, you’d need to carry out additional surveying and site-specific modifications.

Ultimately, having to make edits to these plans defeats the purpose of their being "pre-approved," and the time taken by building authorities to review modifications puts them on par with other ADU plans.

So that brings up the question: Is it cheaper to build out a pre-approved plan or a customized one?

Do Pre-Approved ADU Plans Help Cut Costs?

Most pre-approved plans weren’t designed with the expertise of a builder and therefore aren’t very efficient.

Initial building costs depend on your chosen plan, but it’s safe to assume that making (inevitable) amendments to the pre-approved design will hike up the cost by a few thousand dollars.

Often, structures aren’t designed for weather resistance and installations aren’t standardized. Wet walls, which are necessary for moisture resistance and plumbing purposes in bathrooms, laundry rooms and kitchens, might be overlooked in the plans. Plans might opt for more specific fittings instead of standard-size and easily sourced doors, windows, and shower/tub inserts. Such oversights might create problems for homeowners in the future; you might end up paying a lot for additional maintenance.

On the other hand, when you start with a customizable ADU plan, you are able to plan around the unique features of your site with minimal back-and-forth. This ensures you are working within your budget, making efficient use of labor, and not compromising on the quality of resources.

Let’s take a closer look at the pros and cons of using pre-approved and customizable ADU plans in achieving your architectural vision.

Should You Use Pre-Approved ADU Plans?

If you are able to choose and work off of one that is fairly close to the ADU you envisioned, then yes, you might be able to build out a pre-approved plan with minimal changes. Starting from here might give you a faster and cheaper build than if you were to start with a blank slate.

However, we’ve noticed that these plans aren’t the most practical or resourceful and are likely to require a lot of fixes along the way. They are code-compliant at best, but still require additional work. So, working with a design firm to customize your ADU from A to Z might work out better for you.

Customizable ADU Plans: Are They a Better Fit?

Customizable ADU plans are drawn up with the specifics of your property in mind. They can be tailored to suit your architectural vision while maintaining compliance with building codes.

While pre-approved plans taken off the shelf seem more feasible, adapting them for your needs incurs additional costs. Since recent San Diego housing reforms have made the construction of ADUs more accessible, there are some of the changes you can take advantage of if you choose to plan and build a customizable ADU instead:

  • More flexibility around zoning regulations
  • Faster permit processing, plan review, and site inspection times
  • Allowing Companion and Junior ADUs (JADUs) up to 1200 sq.ft. (an increase from 700 sq.ft.)

Efficiently designed ADU plans save you both time and money in the short and long term.

Building a Customizable ADU: How Can Cottage Help?

We have a variety of small, medium and large ADU floor plans that you can use as inspiration when getting started.

Vanessa - 436 sq.ft | 1 Bed | 1 Bath

Hopkins - 732.5 sq.ft | 1 Bed | 1 Bath

Wallen - 1200 sq.ft | 2 Bed | 2 Bath

Work off of an existing plan and customize it as you wish – add utility spaces, modify your kitchen, or restructure your patio – or start entirely from scratch!

Whichever route you choose, our team is here to guide you.

If customization is what you need, we will take care of the heavy lifting. San Diego’s ADU building regulations are sorted according to municipality area. We handle the planning, designing, and permitting that goes into bringing your ADU to life. We also have a network of reliable and experienced builder partners whom we carefully screen before putting on your project and can be counted on for high-quality work.

The cost of building a customizable ADU is entirely dependent on site conditions and your architectural vision, so it may or may not be more expensive than a pre-approved ADU plan. However, it’s safe to say that the additional cost is worth it in the long run; you’re paying for an efficient and long-lasting build that suits your needs and budget.

Ready to get started on building your ADU? Reach out to our experts!


How much does it cost to build an ADU in San Diego?

Costs can vary over time, but a general rule of thumb is that you can expect to pay a few hundred dollars per sq.ft. The overall cost of your ADU project can be broken down into six main areas: site work, design and engineering work, vertical construction, permitting fees, solar paneling installation, and site-specific requirements. We can provide you with an estimate of the total project cost at the start, so reach out to us for a more accurate breakdown based on your precise location, needs, and lot characteristics.

How much does an ADU permit cost in San Diego?

The cost of permits will vary depending on the scope of your ADU project, the size of your home, its locale and school district, but will likely amount to a few thousand dollars. However, most ADU permitting fees have been waived to speed up the process, saving you thousands on your project!

How long does it take to get an ADU permit in San Diego?

Once you have a design together, the overall permitting process is often roughly 6 months. However, working with Cottage means we can submit and get approvals on your behalf, swiftly.

How big can an ADU be in San Diego?

In the County of San Diego, a detached ADU can be up to 1200 sq.ft.. Meanwhile, a 1 bedroom attached ADU can be up to 850 sq.ft. and a 2+ bedroom attached ADU can be up to 1,000 sq.ft. A 1 bedroom attached ADU can be up to 850 square feet. A 2+ bedroom attached ADU can be up to 1,000 square feet. Other local San Diego jurisdictions, including unincorporated areas within the County of San Diego, can have varying requirements. However, all are subject to statewide ADU-friendly laws.

What is the most expensive part of building an ADU?

Though the cost of building an ADU is dependent on square footage and other factors, construction costs often contribute to 90% of the cost of your project. It also accounts for site work and structural work.

How much does a prefab ADU cost in San Diego?

The starting price of prefab ADUs can vary. As of early 2023, the all-in cost for a prefab ADU in San Diego, including architectural design, permitting, and the prefab unit, ranges between $300-$500 per square foot. Cottage offers high-quality, customizable ADUs. You might be surprised to learn that the cost of a customizable ADU often works out to less than that of a prefab ADU.