By Emily Carson, March 10, 2026

A to Z Homes

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. However, with various terms, building codes, costs, and design choices to navigate, it’s easy to feel overwhelmed. This guide will 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 typically a small secondary living space on a residential property. It is most often used to accommodate family members such as aging parents, adult children, or relatives who require independent living arrangements near the primary dwelling. Many homeowners also utilize granny flats as rental units, guest suites, or flexible spaces for a home office, gym, studio, or extended-stay visitors.

While “granny flat” is not a formal legal term, it has become a colloquial way to refer to any self-contained residential unit located on the same lot as a primary home.

Where Does the Name Granny Flat Come From?

The name is derived from the traditional role these units played in multigenerational households. Before California established statewide accessory dwelling unit (ADU) regulations, homeowners commonly built small backyard units specifically for a parent or grandparent to live nearby without sacrificing privacy or independence. These informal structures were widely referred to as “granny flats,” a term that has persisted even as regulations evolved.

The trend accelerated after 2020, when California enacted comprehensive ADU laws that simplified the process of legally building and permitting these units. Statewide, every single-family property is entitled to a detached ADU of up to 800 square feet (often between 1,000-1,200 square feet), a junior ADU of up to 500 square feet within the primary residence, and conversion ADUs. Multifamily properties may allow even more options.

The increase in multigenerational living, combined with these favorable housing laws, is why the term “granny flat” has reemerged as a viable housing solution.

What Is the Difference Between a Granny Flat and an ADU?

A granny flat is essentially an informal nickname for an accessory dwelling unit (ADU), which is the legal term used in California. In many contexts, these phrases refer to the same type of permitted secondary dwelling, and they are often used interchangeably. To qualify as an ADU, the structure must be at least 150 square feet and equipped with a bathroom and a kitchen or kitchenette.

This legal distinction is important because ADUs are granted specific rights and protections under state law, such as streamlined approval processes, relaxed parking requirements, and the ability to rent the unit long-term.

Legal and Regulatory Considerations for Building a Granny Flat

Understanding what you are actually building is a critical first step in navigating the regulatory landscape. While terms like “granny flat” and “casita” are frequently used, they do not carry legal definitions. Cities regulate particular unit types like accessory dwelling units (ADUs), junior ADUs, tiny homes, and guest quarters, each governed by its own rules for size, kitchen requirements, and permissible uses. Grasping the category your project fits into will guide you in understanding what you can legally build.

Accessory Dwelling Unit Regulations in California

In California, state law allows a single-family property to include one detached ADU (up to 800 square feet), one junior ADU (up to 500 square feet within the main home), and a conversion ADU. Multifamily properties may allow for at least two detached ADUs plus conversion units. Recently, the state laws were amended to permit up to eight detached ADUs, provided they do not exceed the total number of existing units.

ditionally, each city may have more lenient guidelines. For example, San Diego’s regulations permit detached ADUs of up to 1,200 square feet, including two-story units. San Diego’s affordable bonus program also allows for additional detached ADUs if they are occupied by low to moderate income tenants. Other jurisdictions, such as Poway, permit ADUs of up to 1,500 square feet.

What Are Guest Quarters and How Do They Differ from ADUs?

Many cities in Southern California also offer a separate use category known as “guest living quarters.” These structures are similar to ADUs but are governed by different regulations and provide distinct benefits.

• Guest quarters are generally designated for use by family members or non-paying visitors.
• They are usually not permitted to be rented out to the public.
• Some cities allow only limited kitchen facilities, such as a bar sink or kitchenette.
• They may be subject to less stringent size or setback regulations than ADUs.
• In certain jurisdictions, a property can have both an ADU and guest quarters on the same lot.

Since guest quarters are not part of the statewide ADU mandate, each city defines them differently. In some areas, they are practically treated like ADUs, albeit without rental rights, while in others they offer strategic advantages for homeowners who want additional space for relatives but prefer to avoid the full ADU process or kitchen requirements. Therefore, from a multigenerational living perspective, a guest quarter can serve as an excellent alternative to a typical granny flat.

Granny Flat Designs and Floor Plans

Granny Flat Floor Plans

When designing a granny flat, it’s essential to maximize both space and privacy. With extensive experience in this field, many companies can offer a variety of customizable options. Homeowners may choose from pre-designed plans, modify existing designs, or create something entirely new. Some cities even offer pre-approved ADU plans to expedite the permitting process and reduce costs. Working with a specialist in ADUs can help to ensure that your space is functional and efficient, particularly when working with limited square footage.

1 Bedroom Floor Plans

One-bedroom granny flats generally range from 400 to 750 square feet. These layouts typically include a living room, kitchen, bathroom, and a private bedroom. Designs can vary widely from square and rectangular to L-shaped, dependent on the property’s specific layout and the homeowner’s individual preferences.

2 Bedroom Floor Plans

Two-bedroom units are popular for various reasons, including their potential to attract higher rental rates and increase resale value. These layouts often range between 600 and 1,200 square feet, making them a favorite for extended family arrangements or those looking to maximize income potential.

How Much Does a Granny Flat Cost?

Cost to Build a Granny Flat in San Diego

The overall cost of constructing a granny flat can vary significantly based on a variety of factors, including size, materials, site conditions, complexity of utilities, and access to the construction site. Generally, the cost for most detached ADUs in San Diego falls within the range of $300,000 to $500,000. Yet, costs can escalate depending on specific circumstances, such as larger units situated on challenging lots or, conversely, smaller units constructed using pre-approved plans and basic finishes.

Financing Options for Your Granny Flat

Several financing options are available for homeowners looking to build a granny flat, including home equity loans, renovation loans, construction loans, and cash-out refinancing. It’s important to work closely with trusted lenders who can help guide you through the financing process, ensuring you have a comprehensive understanding of your project’s scope from site work to utilities, construction, and finishes.

Property Tax Implications for a Granny Flat

Many homeowners are surprised to discover that adding an ADU does not necessitate a comprehensive reassessment of the entire property. Under Proposition 13, only the new construction value is added to the property taxes, meaning the existing home retains its original assessed value. The ADU is assessed separately based on its construction cost, typically resulting in an increase in annual property taxes of only one to two percent of the ADU’s added value.

Building a Granny Flat: What to Know Before You Start

Building Process

The process of building a granny flat is divided into several key stages. The initial phase involves a feasibility study and custom design, followed by permitting and revisions, and culminating in the construction itself. The construction phase itself includes site preparation, trenching and foundation work, framing, rough mechanical, electrical, and plumbing installation, exterior finishes, insulation and drywall, and finally the installation of interior finishes, with a final inspection concluding the project.

Feasibility as the First Step

While many prospective builders wish to jump straight into design, it’s crucial to conduct a feasibility study first. This essential step determines whether constructing a granny flat on your property is a viable option. A thorough feasibility study involves zoning checks, site analysis, utility mapping, and initial floor plans with 3D renderings to provide an understanding of potential site constraints or utility issues that could arise.

Choosing the Right Granny Flat Builder

Working with Granny Flat Contractors

Partnering with an experienced, licensed team is fundamental to a successful project, as specialized granny flat builders are well-versed in local regulations, permitting, and design challenges. When selecting a builder, look for expertise in ADUs, a robust project portfolio, clear and transparent pricing, realistic timelines, effective communication, milestone-based payments, warranties, and positive reviews. Be cautious of vague quotes, substantial upfront deposits, and insufficient project history.

How to Get Accurate Quotes and Avoid Common Pitfalls with Granny Flat Builders

Different ADU providers operate under varying business models. Some firms function as full-service design-build companies, while others may offer consulting services that subcontract design and construction. It’s vital that quotes encompass all aspects of the project, including feasibility, design, permit management, site preparation, construction, utility connections, and finishing materials. Watch for hidden costs or unrealistic timelines, and ensure a trustworthy contractor provides a comprehensive and accurate quote.

Top Granny Flat Builders in San Diego: Why You Should Choose Local Experts

Since 2020, a number of builders have established themselves as go-to options for ADUs in San Diego. Their projects range from stylish and functional designs to innovative construction techniques that set industry standards. Homeowners can explore their work online, complete with testimonials and project highlights, to better understand what it’s like to collaborate with these professionals.

For more information on granny flats and to explore detailed plans and designs, feel free to visit A to Z Homes.

Disclaimer: This information is for educational purposes only and should not be considered legal or financial advice. Please consult with a professional in your area for personalized guidance.