Curious about adding a granny flat to your property? Perhaps you’re looking to create a space for family, generate rental income, or expand your home’s flexibility. Building a granny flat (also called an accessory dwelling unit or ADU) can be a smart move. But with different terms, building codes, costs, and design choices to navigate, it’s easy to feel overwhelmed. We’ll walk you through what a granny flat is, how it differs from other unit types, and everything you need to know to plan, design, and build one with confidence… especially here in San Diego, where laws make ADUs more accessible than ever.
What Is a Granny Flat?
Granny Flat Meaning
A granny flat is a small secondary living space on a residential property. It is most commonly used to house family members such as aging parents, adult children, or relatives who need nearby but independent living arrangements. Many homeowners also use granny flats as rental units, guest suites, or flexible space for a home office, gym, studio, or extended-stay visitors.
Although “granny flat” is not a legal term, it has become a familiar way to describe any self-contained residential unit located on the same lot as a primary home.
Where Does the Name Granny Flat Come From?
The name comes from the traditional role these units played in multigenerational households. Before California created statewide accessory dwelling unit (ADU) standards, homeowners often built small backyard units specifically so a parent or grandparent could live close by without sacrificing privacy or independence. These informal structures were widely referred to as “granny flats,” a name that stuck even as regulations evolved.
The trend accelerated after 2020, when California enacted sweeping ADU laws that made it much easier to legally build and permit these units. Statewide, every single family property is entitled to a detached ADU of up to 800 square feet (and often between 1000-1200 SF), a junior ADU of up to 500 square feet within the primary residence, and a conversion ADU. Multifamily properties can add even more (keep reading for details).
The rise in multigenerational living, combined with these pro-housing laws, is why the term “granny flat” has reemerged as a mainstream housing solution.
What Is the Difference Between a Granny Flat and an ADU?
A granny flat is simply an informal nickname. An accessory dwelling unit (ADU) is the legal term. In California, these two phrases usually refer to the same type of permitted secondary dwelling and these terms are used interchangeably. To qualify as an ADU, the structure must be at least 150 square feet and include a bathroom and a kitchen or kitchenette.
This legal definition matters because ADUs enjoy specific rights and protections under state law, such as ministerial approval, relaxed parking requirements, and the ability to rent the unit long-term.
Legal and Regulatory Considerations for Building a Granny Flat
Before diving into regulations, it helps to clarify what you are actually building. “Granny flat” and “casita” are common nicknames, but do not carry legal definitions. Cities regulate specific unit types like accessory dwelling units (ADUs), junior ADUs, tiny homes, and guest quarters, each with its own rules for size, kitchens, and how the space can be used. Understanding which category your project falls into is the first step in learning what you can build and navigating the legal landscape.
Accessory Dwelling Unit Regulations in California
In California, statewide law permits a single-family property to include one detached ADU (up to 800 sq ft), one junior ADU (up to 500 sq ft within the main home), and a conversion ADU. Multifamily properties are allowed even more: at least two detached ADUs plus conversion units. Recently the state laws changed to allow up to eight detached ADUs, so long as they do not exceed the number of existing units… though it would be a stretch calling this kind of development ‘granny flats!”
Each city may offer more generous guidelines. San Diego ADU regulations for instance allow for detached ADUs up to 1,200 square feet and allows two-story units. Through its affordable bonus program, some City of San Diego properties qualify for additional detached ADUs if they are rented to low to moderate income tenants. Some jurisdictions are even more lenient with ADU size. For example, the city of Poway allows ADUs up to 1,500 square feet.
See our ADU regulations to see the rules that pertain to granny flats in your city.
What Are Guest Quarters and How Do They Differ from ADUs?
Many cities in Southern California also allow a separate use category called “guest living quarters.” These structures look similar to ADUs but follow different rules and offer different benefits.
Key characteristics of guest quarters:
- They are typically limited to use by family members or non-paying guests.
- They usually cannot be rented to the public.
- Some cities prohibit full kitchens, allowing only a bar sink or kitchenette.
- They may have more lenient size or setback rules than ADUs.
- In some jurisdictions, a property can have both an ADU and guest quarters on the same lot
Because guest quarters are not part of the statewide ADU mandate, every city defines them differently. In some areas they are treated almost like ADUs without rental rights. In others they offer strategic advantages for homeowners who want space for relatives but do not want to commit to the full ADU process or kitchen requirements.
Because these units cannot usually be rented, they have different financial implications than ADUs. But from a multigenerational living standpoint, they can be an excellent fit for a classic granny flat.
See our guide to guest living quarters, which includes a city-by-city list of what is allowed.
Granny Flat Designs and Floor Plans
Granny Flat Floor Plans
When designing a granny flat, it’s crucial to maximize both space and privacy. At SnapADU, we’ve designed over 100 units. You can choose from our past client plans, modify an existing design, or start fresh with something custom. Some cities offer pre-approved ADU plans to speed up permitting and reduce costs. Working with a team that specializes in ADUs ensures your space functions efficiently within limited square footage.
1 Bedroom Floor Plans
One-bedroom granny flats usually range from 400 to 750 square feet. These units include a living room, kitchen, bathroom, and a private bedroom. Layouts vary widely from square and rectangular to L-shaped depending on the lot shape and preferences. Explore one bedroom granny flat plans.
2 Bedroom Floor Plans
Two-bedroom layouts are popular for many reasons. They attract higher rents and can significantly boost resale value. These plans typically range from 600 to 1,200 square feet. Homeowners often choose two-bedroom units for extended family living or to increase income potential.
We’ve designed multiple two-bedroom, three-bedroom, and even four-bedroom ADUs. All are available on our website to view or customize. Explore two bedroom granny flat plans, three bedroom granny flat plans, and four bedroom granny flat plans.
How Much Does a Granny Flat Cost?
Cost to Build a Granny Flat in San Diego
The total cost of building a granny flat depends on size, finishes, site conditions, utility complexity, and access. Most detached ADUs in San Diego cost between $300,000 and $500,000 all-in. However, this can vary. A large unit on a challenging lot might exceed this range, while a smaller ADU using a pre-approved plan and modest finishes may fall below it. You can check out our ADU Costs page to read more about how costs vary with size and other factors.
Financing Options for Your Granny Flat
There are several ways to finance a granny flat, including home equity loans, renovation loans, construction loans, and cash-out refinancing. At SnapADU, we partner with trusted lenders who can help guide you through the process.
It’s important to understand your full project scope early on. Site work, utilities, construction, finishes, design, and permitting all affect your budget. Careful planning ensures your project stays on track.
Read more about ADU Financing.
Property Tax Implications for a Granny Flat
Many homeowners are surprised to learn that adding an ADU does not trigger a reassessment of the entire property. Under Proposition 13, only the new construction value is added. This means the existing home keeps its original assessed value. The ADU is assessed separately based on its construction cost. The increase in annual property tax is typically one to two percent of the ADU’s added value.
Read more about ADU tax implications.
Building a Granny Flat: What to Know Before You Start
Building Process
Building a granny flat involves several stages. First comes a feasibility study and custom design. Then you’ll move into permitting and revisions. After that, construction begins.
The construction phase includes site preparation, trenching and foundation, framing, rough mechanical, electrical, and plumbing, followed by exterior finishes, insulation and drywall, and finally interior finishes. A final inspection completes the process.
Check out our construction process guide.
Feasibility as the First Step
While a lot of folks want to start with design, there is a crucial first step. A feasibility study is essential to determine if building a granny flat on your property makes sense. At SnapADU, our feasibility studies include zoning checks, site analysis, utility mapping, and concept floor plans with 3D renderings.
This upfront work helps avoid costly surprises later, like unexpected site constraints or utility issues. With a solid understanding of the scope and cost, you can move forward with confidence.
Learn more about our feasibility studies.
Choosing the Right Granny Flat Builder
Working with Granny Flat Contractors
Partnering with a licensed and experienced team is key. Specialized granny flat builders understand local rules, permitting, and design challenges. At SnapADU, we offer an all-in-one service to make the process smoother.
When choosing a granny flat builder, look for specialization in ADUs, a robust project portfolio, local knowledge, transparent pricing, realistic timelines, strong communication, milestone-based payments, warranties, and positive reviews. Avoid red flags like vague quotes, large upfront deposits, and a lack of project history. Check out our guide to choosing your ADU provider.
Read more about why remodel & home improvement companies sometimes struggle with ADUs.
How to Get Accurate Quotes and Avoid Common Pitfalls with Granny Flat Builders
Different ADU providers operate in different ways. SnapADU is a full-service design-build firm that handles everything in-house. Others may be consultants who outsource construction and design.
Make sure quotes include everything: feasibility, design, permit management, site work, vertical build, utility hookups, and finish materials. Look out for hidden costs, mismatched scopes, or unrealistic timelines. A trustworthy contractor will give you a clear and complete picture. You can read more in our guide to comparing ADU quotes.
Top Granny Flat Builders in San Diego: Why SnapADU?
Since 2020, SnapADU has been San Diego’s go-to ADU expert. We’ve completed over 100 granny flats and continue to lead the region in quality and innovation. Our clients value our clear communication, predictable outcomes, and detailed project tracking.
Visit our projects page to see real examples, from feasibility through construction. You’ll also find video testimonials and customer reviews that show what it’s like to work with us… so you can avoid the challenges many homeowners face when building an ADU.
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