What Is a Granny Flat? Definition, Types, and How They’re Used

TL;DR: What Is a Granny Flat

• A granny flat is a small, self-contained living unit on the same property as a main home, typically with its own kitchen, bathroom, and entrance.
• Granny flats are commonly used for multigenerational living, rental income, or flexible space such as a guest house or home office.
• They can be built as detached structures, garage conversions, attached additions, or interior units, depending on property layout and local rules.
• In California, many granny flats qualify as ADUs or JADUs, which affects size limits, permitting, and design requirements.
• Costs generally range from $100,000 to $300,000+, depending on size, construction type, and finishes.
• This article covers definitions, types, costs, regulations, design options, timelines, and builder selection.

Key Takeaways

• As housing prices rise in San Diego and multigenerational living becomes more common, homeowners are increasingly turning to accessory dwelling units, or ADUs, for added flexibility, building what is known as a granny flat or in-law suite.
• Though these granny flats may be used initially to provide affordable housing for extended family, they offer a wide range of benefits throughout their lifetime, including as a source of potential rental income or extra space for guests.
• Granny flats can be new builds, garage conversions, or even built within the walls of the existing home, which is known as a junior ADU or JADU, and customized to fit various needs.
• Granny flats typically cost between $150,000 and $300,000 to build, and most homeowners find they’re worth the investment, as they allow the same single-family property to work for multiple generations.

A well-placed granny flat transforms underutilized yard space into high-value, income-generating square footage—without compromising your main home.

What Is A Granny Flat?

A granny flat is a flexible secondary living space built on the same property as a main home, commonly used for multigenerational living, rental income, or guest accommodations. These units can be detached, converted from garages or interior space, or built as attached additions, depending on property layout and local rules. In California, many granny flats qualify as ADUs or JADUs, which affects size limits and permitting. Costs typically range from $100,000 to $300,000+, based on size, construction type, and finishes. This article explains types, costs, regulations, design options, timelines, and how to choose a builder.

What is a granny flat is a small, self-contained living unit built on the same property as a primary home, typically including a kitchen, bathroom, bedroom, and separate entrance. In many cases, a granny flat qualifies as an accessory dwelling unit (ADU) and must meet local size and zoning rules. Planning, permitting, and construction usually take several months, depending on design complexity and approval timelines.

But are granny flats the same thing as accessory dwelling units (ADUs)? Not necessarily.

Many times, granny flats are built within the walls of an existing home. Homeowners may convert a basement, for example, and include a kitchenette, a small living area, and a bedroom, but have the granny flat share a bathroom with a main house. This wouldn’t be considered an ADU, but rather a JADU, or junior ADU.

To be considered a true ADU, a structure must:

• Be no larger than 1200 square feet
• Contain a kitchen with cooking appliances and sink (appliances like hot plates don’t count)
• A bathroom with a toilet, a sink, and a shower or tub

To read more about what qualifies as an ADU, you can read our separate blog post here.

Granny flats fit naturally into San Diego neighborhoods—make sure your design complements the surrounding homescape.

Granny Flat vs ADU vs JADU: What’s the Difference?

A granny flat is a general term for a secondary living space, while ADU (Accessory Dwelling Unit) and JADU (Junior ADU) are specific legal classifications defined by California law.

Common distinctions include:

• Granny flat: A use-based term describing how the space is lived in
• ADU: A fully independent unit with a kitchen, bathroom, and separate entrance
• JADU: A smaller unit within an existing home that may share a bathroom

Quick Answers

• Is every granny flat an ADU?
  No. Only units that meet legal ADU requirements qualify.
• Can a JADU be called a granny flat?
  Yes, based on use—but legally it’s still a JADU.
• Why does the distinction matter?
  Permits, size limits, and rental rules depend on classification.
• Which is easiest to build?
  JADUs are typically the simplest and least expensive.

Key differences at a glance

Common Names for Granny Flats (Apartment, Unit, Suite Explained)

Granny flats are known by many names, often depending on region, layout, or intended use rather than strict legal differences.

You may hear them called:

• Granny apartment
• Granny unit
• Granny suite
• In-law unit or in-law suite
• Backyard cottage or guest house

Despite different names, these terms usually refer to a secondary living space on the same property as a main home.

Quick Answers

• Do these names change legal requirements?
  No. Zoning and permitting depend on structure and features, not the name.
• Which term should homeowners use?
  Use the legal term (ADU or JADU) for permits; informal terms are fine otherwise.
• Is “granny apartment” the same as a rental unit?
  It can be, but not always.
• Why are there so many names?
  Usage evolved before modern ADU laws existed.

The Different Types of Granny Flats

There are several types of granny flats to consider. The differences between each have to do with the method of construction.

• Detached Granny Flats: This describes a new, standalone housing unit constructed from scratch on the same property as a single-family home, usually in the backyard. These can range in size from 300 square feet to 1,200 square feet.
• Attached Granny Flats: These are independent living units that are built, essentially, as an addition to the primary dwelling. By California law, attached ADUs can be up to 50% of the existing home’s square footage, or 1,200 square feet—whichever is less.
• Garage Conversion Granny Flats: An unused detached garage is converted into a self-contained living area. These offer the same functionality and return on investment as a new, detached structure but for as little as half the cost.
• Junior ADU Granny Flats: Basements or extra bedrooms are converted into an in-law suite that may or may not contain its own bathroom.

Sometimes, people compare granny flat ADUS to tiny homes, but these two structures are different. The biggest difference is that ADUs have a permanent foundation, while a tiny home can be on wheels. You can read more about tiny homes vs ADUs here.

Detached units offer the greatest design freedom—plan for future rental use by including separate utilities and private access early in construction.

How Building a Granny Flat Works (High-Level Overview)

Building a granny flat typically involves planning, permitting, and construction, with the exact process varying by property type and local regulations.

At a high level, most projects include:

1. Property feasibility and zoning review
2. Design and layout planning
3. Permit submission and approvals
4. Construction and inspections
5. Final sign-off and occupancy

Timelines can range from several months to over a year, depending on complexity and jurisdiction.

Quick Answers

• Do I need permits to build a granny flat?
  Yes, in almost all cases.
• How long does construction take?
  Often 4–6 months once permits are approved.
• What slows projects down most?
  Permitting and utility upgrades.
• Can I live on the property during construction?
  Usually, yes.

Granny Flat Rules in California and San Diego

California law strongly supports ADUs, but local jurisdictions like San Diego still enforce specific zoning and development standards.

Common regulatory factors include:

• Maximum unit size and height
• Setback requirements from property lines
• Parking rules (often reduced or waived)
• Utility connections and impact fees

San Diego generally allows at least one ADU on most single-family lots, with additional options in some zones. As of 2026, California continues to streamline ADU and JADU regulations to expand housing options and reduce local barriers. Most cities, including San Diego, must allow compliant granny flats that meet state standards, even if local design rules apply. While specific requirements can vary by jurisdiction, statewide law generally prevents cities from prohibiting legally compliant ADUs outright.

Quick Answers

• Are granny flats legal everywhere in California?
  Mostly, but details vary by city.
• Do HOAs affect granny flats?
  HOAs can restrict design but usually can’t ban ADUs outright.
• Is owner occupancy required?
  In many cases, no.
• Do rules change often?
  Yes—state laws are updated regularly.

Advantages and Disadvantages of Granny Flats

Granny flats are a huge investment of time, money, and energy. But is it worth it? Here are some things to consider:

Advantages of building a granny flat include:

• Space for Family Members: Granny flats provide housing for elderly parents or even adult children, without having to move and contend with the expensive San Diego real estate market.
• Rental Income: While you may be building the structure as a place to house aging relatives, in time, you may be able to lease the unit to tenants to generate tens of thousands in passive income.
• Flexible Living Arrangements: If you don’t choose to rent out your granny flat, you can still repurpose it as a guest house, art studio, or home office
• Property Value: Granny flats increase overall property value, sometimes by as much as 35%.

Disadvantages of building a granny flat include:

• Building Costs: Permitting and construction of these units can be as much as $300,000 or more.
• HOA Restrictions: While homeowners associations can’t ban the building of ADUs outright, they can place restrictions on appearance and maximum size.
• Loss of Yard Space: Building a second structure can take away valued outdoor space, especially on a smaller lot.

ADUs can boost property value by up to 35%, especially in high-demand areas—consider location-specific ROI before starting your build.

Is a Granny Flat a Good Investment?

For many homeowners, granny flats provide both lifestyle and financial returns.

Potential benefits include:

• Rental income
• Increased property value
• Long-term housing flexibility

Returns depend on location, build cost, rental demand, and local regulations.

Quick Answers

• How long until a granny flat pays for itself?
  Often 10–20 years, depending on rent.
• Do granny flats increase taxes?
  Usually, yes.
• Are they easier to rent than apartments?
  In many markets, yes.
• Is resale value guaranteed?
  No, but increases are common.

Key Granny Flat Solutions: Sizes and Amenities

While San Diego allows ADUs to be as large as 1,200 square feet, most of the granny flat ADUs we build fall between 400 and 1000 square feet.

They also typically contain amenities like:

• At least 1 bedroom (though some homeowners opt for 2 bedrooms for more of a single-family feel)
• A full kitchen
• A full bathroom with a walk-in shower
• Separate entrance for privacy
• In-unit laundry
• Wider hallways and one-story living

Because many of these features, like a walk-in shower and wider hallways, have to do with long-term accessibility, granny flat ADUs often benefit from custom designs as opposed to prefab or manufactured ADUs.

Granny Flat Floor Plans: 1-Bedroom vs 2-Bedroom Options

Granny flat floor plans commonly range from one to two bedrooms, with layout choices affecting cost, comfort, and flexibility.

Typical differences include:

• 1-bedroom granny flats: More compact, lower cost, ideal for one occupant
• 2-bedroom granny flats: Higher cost, better for families or rentals

Quick Answers

• Is a 2-bedroom granny flat worth it? Often yes for rental or resale value.
• Can I add a second bedroom later? Usually difficult without major changes.
• Do 2 bedrooms increase permits complexity? Sometimes, depending on size limits.
• What layout is most popular? Open-concept living with separated bedrooms.

The Cost to Build a Granny Flat

The average cost to build a granny flat in San Diego or similar markets ranges from $100,000 to $300,000, depending on the building type, square footage, and finishes.

Typically, we tell homeowners to anticipate spending:

• $100,000 – $150,000 for a JADU
• $150,000 – $250,000 for a garage conversion ADU
• $300,000+ for a new, detached ADU

Where your build falls within this range depends on how big your ADU is and what finish you choose to include.

The good news is that the state of California has made many financing options as a way to incentivize the construction of ADUs and combat the state’s housing crisis.

Site prep and foundation work often represent a significant chunk of your ADU budget—plan accordingly and pad your contingencies.

Granny Flat Plans and Prices: What to Expect

The cost of a granny flat depends on size, construction type, finishes, and site conditions.

Typical price ranges include:

• Interior conversions or JADUs: lower cost range
• Garage conversions: mid-range
• Detached new builds: highest cost range

Quick Answers

• Why do prices vary so much?
  Utilities, design choices, and site work drive costs.
• Are plans included in the price?
  Sometimes, but not always.
• Can I reuse stock plans?
  In some cases, yes.
• Do prices include permits?
  Not always—confirm upfront.

Prefab vs Custom Granny Flats: Pros, Cons, and Costs

Granny flats can be built using prefab (modular) construction or custom on-site builds, each with tradeoffs.

Prefab options offer:

• Faster build times
• Lower base costs
• Limited customization

Custom builds offer:

• Full design flexibility
• Better long-term durability
• Higher upfront costs

Quick Answers

• Are prefab granny flats cheaper?
  Often upfront, but hidden costs can apply.
• Do prefab units qualify as ADUs?
  Yes, if code-compliant.
• Which has better resale value?
  Custom builds usually perform better.
• Is prefab allowed everywhere?
  Not always—site access matters.

Renovating vs Building a New Granny Flat

Homeowners may choose between renovating an existing space or building a new structure.

Renovations often include:

• Basement conversions
• Garage conversions
• Interior reconfigurations

New builds offer:

• Fewer layout compromises
• Higher costs
• Greater long-term flexibility

Quick Answers

• Is renovating always cheaper?
  Not if major upgrades are required.
• Which option is faster?
  Renovations are usually quicker.
• Do renovations qualify as ADUs?
  Only if they meet all requirements.
• Which option adds more value?
  New builds often add more.

Regulatory Factors to Consider Before Building a Granny Flat

Though California does incentivize the building of ADUs, there are still state and local regulations that need to be followed to get your building permit approved. These include:

• ADU setbacks from property lines
• Parking requirements (which may be waived if you’re close to public transit)
• Minimum and maximum size limits
• Accessibility requirements for certain unit types

Each jurisdiction has its own building department and permitting process, so check with your local agency or partner with an experienced ADU designer & builder.

Always verify setback and zoning regulations before framing—changes at this stage are costly and can delay your build.

Designing Your Ideal Granny Flat: Floor Plans and Inspirations

When it comes to granny flats, design aesthetics can run the gamut from ultra modern to California coastal to traditional cottage. Whatever the aesthetic, granny flats contain common design features including:

• Open-concept living
• L-shaped or U-shaped kitchens to maximize cabinet space
• High ceilings and light color schemes
• Built-ins and other custom storage solutions

As for how this comes to life as a floor plan, we have many free floor plans you can view. Some popular 1-bedroom plans are here, and 2-bedroom plans are here.

Design isn’t just aesthetic—it’s strategic. Use natural light, ceiling height, and open-concept layouts to make smaller spaces feel expansive.

How to Select Your Granny Flat Builders

The most important thing when choosing a granny flat builder is to find someone who’s experienced with ADUs and granny flats specifically, as opposed to a general contractor. ADUs come with their own set of rules, regulations, and design best practices that a single-family home builder may not be familiar with.

Other than that, meet with all potential builders, and look for one that has:

• Positive customer reviews and a solid track record
• Transparent pricing and clear contracts
• Adequate insurance
• A clear communication style

These qualities will help you avoid hidden costs and keep your ADU project on time and on budget.

ADUs require specialized expertise—partner with a builder who understands local permitting, ADU-specific codes, and long-term property value strategy.

Common Granny Flat Questions and Terminology

This section captures frequent questions, alternate spellings, and informal terms users search for.

Quick Answers

• What does “flat grannies” mean?
  An informal or incorrect phrasing of granny flats.
• Are spelling variations important?
  No, they all point to the same concept.
• Is “granny flat” outdated language?
  It’s still widely used, though ADU is more legalistic.
• Do all granny flats house seniors?
  No—many are rentals or guest units.

Building Your Dream Granny Flat with Better Place Design Build

At Better Place Design & Build, we specialize in designing and constructing backyard homes, granny flats, and accessory structures that add real value to your single-family home. Whether you’re looking to house extended family or generate additional income, we’re here to help you every step of the way.

Ready to start your own granny flat project? Schedule a free consultation. We’ll walk you through the process—from building plans to move-in day—with honesty and clarity.

The finish stage is where craftsmanship matters most—opt for durable, low-maintenance materials that enhance curb appeal and resale value.