Are Pre-Approved ADU Plans Right for You in San Diego?

What Are Pre-Approved ADU Plans?

Pre-approved ADU plans are standardized Accessory Dwelling Unit (ADU) construction plans that have already been reviewed by local building departments for compliance with current building codes. In San Diego County, “the plans are approximately 85% complete and require you to fill in project-specific information” (San Diego County). This means the floor plan and structural details are city-approved, but property owners still need to complete a building permit application with site-specific items like a plot plan, utility connections, and Title 24 energy code compliance.

For homeowners, these pre-approved construction plans are an effective option because they are free or low-cost, reducing the cost of preparation compared to custom designs. They’re available through both the County of San Diego ADU Plans and the City of San Diego’s Companion Unit Program. By starting with these permit-ready blueprints, property owners can streamline the building permit issuance process, saving time and moving more quickly toward construction of their secondary unit or backyard cottage.

Pre-approved ADU construction plans save time, but don’t forget you’ll still need site-specific items like energy code compliance and utility hookups.

Benefits of Using Pre-Approved Plans

Pre-Approved Accessory Dwelling Unit (ADU) Plans give San Diego homeowners a faster, more affordable path to building secondary units such as backyard cottages or granny flats. Because these pre-approved ADU construction plans are already checked for compliance with current City regulations and development standards, you can skip much of the back-and-forth during the building permit process.

• Cost savings – Pre-approved plans are free or low-cost, saving property owners an estimated $8,000–$25,000 in design fees compared to custom ADU design work (NestADU).
• Faster permitting – Under California AB 1332, cities must review building permit applications using pre-approved plans within 30 days, streamlining the production of housing (City of San Diego).
• City-approved confidence – Since these permit-ready blueprints are vetted by a licensed professional and reviewed against the ADU Handbook, there’s less risk of rejection for code basics.
• Best for simple projects – These plans are most effective for flat lots, square foot studios, or straightforward living units where homeowners want a quick and budget-conscious option (San Diego County).

By using Pre-Approved Accessory Dwelling Unit Plans, property owners can take advantage of a streamlined production process that speeds up building permit issuance while avoiding the additional cost of custom architectural design.

Pre-approved ADU plans save thousands in design fees, but maximize value by pairing them with efficient layouts and compliance from day one.

Challenges of Pre-Approved Plans

While Pre-Approved ADU Construction Plans Programs can save property owners time and money, they also come with limitations that should be considered before moving forward.

• One-size doesn’t fit all – Standard plans may not align with the unique conditions of a property, such as irregularly shaped lots, sloped terrain, or relation to property lines. In these cases, additional engineering or design options are required, which can increase the overall cost.
• Limited customization – According to Los Angeles County, “Plans cannot be customized after pre-approval” (LA County Public Works). Even minor changes, like adjusting bathroom facilities or moving a window, can trigger a full re-review under the Pre-Approved ADU Submittal Checklist, eliminating the fast-track benefit.
• Hidden costs – Pre-approved ADU plan sets do not include site-specific requirements like energy certification, utility hookups, or Title 24 energy calculations. Property owners will still need to provide a site plan, sprinkler design, and possibly agency clearance from Environmental Health or the relevant water district, which can add $5,000–$15,000 or more to the project.
• Real-world example – A San Diego homeowner in La Mesa selected a pre-approved plan but had a sloped lot. By the time the design was adapted by their Building Team, the total cost exceeded what a custom plan would have required, wiping out the expected savings (Better Place Design & Build).

Pre-approved ADU plans are best suited for flat lots, straightforward secondary units, or smaller square foot studios. For more complex properties or homeowners who want flexibility in design and integration with a primary residence, these challenges often outweigh the initial benefits.

Pre-approved ADUs often require extra site-specific work—budget for hidden costs like energy compliance and utility hookups to avoid surprises.

Custom ADU Plans as the Alternative

Custom ADU plans give homeowners more control and flexibility compared to pre-approved designs. Instead of working within the limits of a standard plan, you can tailor every aspect of the design to your primary dwelling, lot size, and long-term goals.

• Tailored to your lot and needs – A custom plan is drawn specifically for your property’s square feet, slope, setbacks, and relation to property lines, ensuring a precise fit. This helps avoid costly modifications later (Better Place Design & Build).
• Design flexibility – With custom plans, you can incorporate Junior Accessory Dwelling Units (JADUs), multi-bedroom layouts, or efficiency kitchen and solar design features that aren’t possible with cookie-cutter blueprints.
• Future-proofing – Custom ADUs can be designed for multigenerational living, rental income, or permanent facilities that support accessibility and aging in place, making them adaptable as your needs evolve.
• Property value impact – A well-designed, site-specific ADU that complements your primary residence adds more resale value than a standard design, since it integrates seamlessly with the home and adheres to development standards (Metropolis Mag).

While custom ADUs involve a higher upfront investment, for many City residents they provide a smoother building permit process, stronger long-term value, and a space designed to meet both current and future needs.

Cost & Timeline Comparison (Pre-Approved vs Custom)

When deciding between pre-approved and custom ADU plans, cost and timeline are usually the biggest considerations. Pre-approved plans reduce upfront expenses and speed up the permit process, while custom designs may take longer but often deliver better long-term value.

Timeline Comparison

• Pre-approved plans – Can save 6–8 weeks in design time and benefit from a 30-day permit review mandate under AB 1332 (City of San Diego).
• Custom plans – Typically require 4–8 weeks for design plus 6–10 weeks for permit review, but may avoid delays later by being fully tailored to the property.

Cost Breakdown

Who Should Use Pre-Approved vs Custom Plans?

Choosing between Pre-Approved ADU Construction Plans and a custom design depends on your property conditions, budget, and long-term goals. Both options can help property owners build Accessory Dwelling Units, but the right fit varies by lot type and future needs.

Pre-Approved Plans Are Best For:

• Standard, flat lots – Optimized for straightforward properties without challenging slopes, unusual property restrictions, or setbacks.
• Budget-sensitive homeowners – These free or low-cost designs save $8,000–$25,000 compared to custom work and simplify the Pre-Approved ADU Submittal Checklist.
• Basic ADU needs – Ideal if you want a functional secondary unit, law unit, or granny flat for family or rental use without focusing on unique design options.
• Speed-focused projects – Applications using Pre-Approved ADU Programs must be reviewed within 30 days under California AB 1332, helping streamline the building permit issuance process.

Custom Plans Are Best For:

• Complex sites – Properties with slopes, irregular shapes, or inadequate lot coverage allowance often require tailored layouts and complete building permit research.
• Long-term flexibility – Custom ADUs can be designed for multigenerational living, Junior ADUs, or efficiency kitchens that evolve with future needs.
• Style-conscious homeowners – Custom plans allow integration with your primary residence or neighborhood character, avoiding conflicts with HOA rules or municipal codes.
• Higher ROI goals – A tailored ADU designed to complement the main home adds more property value and appeals to buyers because it looks like a permanent facility rather than a cookie-cutter addition.

Flat, standard lots often benefit from pre-approved ADUs, while properties with slopes or restrictions are better suited for custom plans.

Where to Find Pre-Approved ADU Plans (Resources)

Homeowners across California can access Preapproved ADU Programs that make it easier to start building Accessory Dwelling Units without beginning from scratch. These resources provide permit-ready plan sets that follow local development standards and can streamline the building permit process.

• County of San Diego (Unincorporated Areas) – The county offers dwelling unit building plans that may be used for ADUs or single-family homes. These plans are about 85% complete and require property owners to fill in site-specific details, such as relation to property lines or adequate lot coverage allowance, using the Pre-Approved ADU Submittal Checklist (County of San Diego).
• City of San Diego (ADU Resources) – The city’s Companion Unit Program accepts pre-approved ADU construction plans from other jurisdictions, including the County, Chula Vista, and Encinitas. This program helps City residents meet property restrictions while expanding housing opportunities through Junior ADUs and other design options (City of San Diego).
• Los Angeles County – Standard ADU Plans Catalog LA County provides a catalog of permit-ready designs for ADUs of different sizes and layouts. These Pre-Approved Accessory Dwelling Unit Plans are free to the public and reviewed for compliance with current building codes, making them an effective option for homeowners looking to streamline approval (LA County Planning).
• Siskiyou County (Northern California) – Siskiyou County’s Community Development Department publishes a range of pre-approved plan sets in different styles such as Craftsman, Farmhouse, and Modern. These can be adapted into ADUs, small homes, or backyard cottages, and are offered free, though printing may involve an additional cost (Siskiyou County).

By using resources from these Preapproved ADU Programs, homeowners can save time, avoid unnecessary design fees, and work within city and county standards for building secondary units that complement a primary dwelling.

Many California cities now publish catalogs of pre-approved ADU plans—compare options locally before starting your project to streamline approval.

Real Talk: Our Recommendation

San Diego’s Pre-Approved ADU Construction Plans Program is an important step toward making Accessory Dwelling Units (ADUs) more accessible, giving property owners an affordable and faster way to add living space to a primary dwelling. Still, in our experience, homeowners with unique property restrictions, long-term goals, or higher resale value expectations usually benefit more from custom ADU design.

While Pre-Approved ADU Submittal Checklists and permit-ready blueprints can save money upfront, custom ADUs typically provide greater flexibility, stronger property value, and fewer project delays since they are designed around your lot’s size, slope, and neighborhood standards. If speed and savings are your top priorities, pre-approved ADUs may be an effective option. But if you want an ADU that’s future-proof, energy-efficient, and seamlessly integrated with your home, a custom plan is the smarter investment—whether you’re in San Diego, Central Contra Costa, or even areas like Placer County or the City of Santa Barbara where unique site conditions and historic resources can add complexity.

👉 Ready to get started? Schedule your free consultation with Better Place Design & Build today, and let’s create the ADU that fits your vision and your property.

Pre-approved ADUs work for simple projects, but for long-term value and flexibility, custom plans usually deliver the smarter investment.