800 Sq Ft ADU Guide: Floor Plans, Costs, 1 & 2-Bedroom Options, and Pre-Approved Designs

Editor

Ori Gura is the founder of LA Family Builders and an expert in accessory dwelling units (ADUs). With years of experience in the construction industry, Ori leads a team that specializes in designing and building custom ADUs. He is passionate about helping homeowners maximize their property’s potential through innovative, efficient, and high-quality building solutions.

An 800 sq ft ADU is like the Swiss Army knife of home additions—spacious but not sprawling, efficient without feeling cramped. Typically sized at 20 ft x 40 ft or 25 ft x 32 ft, these ADUs come in all shapes and layouts, from cozy one-bedroom retreats to family-friendly two-bedroom setups. Open-concept designs? Check. Clever storage solutions? You bet. Some even toss in a patio or loft to sweeten the deal.

So, how much are we talking? In California, costs range from $160,000 to $320,000—though $200,000 seems to hit the sweet spot, assuming you’re working with the median cost of $250 per square foot. Of course, where you are makes a big difference. San Francisco? Brace yourself for $379,200. Los Angeles? A more “reasonable” $236,800.

One-bedroom units in this size usually work for 2-4 people—perfect for a couple or a small family. Add a second bedroom, and you’ve got room for up to six (or a goldmine for potential rental income). Plus, with pre-approved plans popping up in places like San Jose, San Diego, and LA, you can shave weeks—and dollars—off the timeline.

Whether you’re housing in-laws, generating income, or finally carving out that dream studio space, these units are multitaskers. Throw in energy-efficient features, and you’re not just saving the planet—you’re saving up to $8,000 over 10 years.

What Are 800 Sq Ft ADU Floor Plans?

Let’s break it down: an 800 sq ft ADU is bigger than your average tiny house (under 400 sq ft) but snug compared to a standard single-family home. It’s a “just-right” Goldilocks situation—small enough to save on construction costs but big enough for real living. Dimensions like 20 ft x 40 ft or 25 ft x 32 ft make the most of your lot without hogging it all.

HUD defines ADUs as self-contained living units, detached from the main home but fully equipped. At this size, you’re getting flexibility—separate rooms, dedicated spaces—and none of the downsides of feeling too cramped. Compared to a 500 sq ft unit, there’s just more breathing room. And compared to a 1,000 sq ft ADU? It’s cheaper and usually easier to get past zoning restrictions.

Popular Layouts for 800 Sq Ft ADUs

The beauty of an 800 sq ft unit is how versatile it is. Here are a few crowd-pleasers:

  • One-bedroom layouts: A cozy setup with room for 2-4 people. Bedroom, bathroom, kitchen, living area—simple and functional.

Two-bedroom designs: These pack in space for up to six people, making them ideal for families or as rental properties.

  • L-shaped configurations: Great for optimizing outdoor space—think patios, decks, and natural flow from inside to out.

Interesting fact: According to the National Association of Home Builders, a typical two-bedroom apartment averages 1,006 sq ft. So, an 800 sq ft ADU? It’s hitting nearly the same functionality in a much smaller footprint.

Garage conversions are also a hit in this category. These often give you funky, creative layouts that blend existing structures with new additions—and sometimes create unique outdoor spaces in the process.

Real-World Examples of 800 Sq Ft ADU Floor Plans

When it comes to 800 sq ft ADUs, real-world examples illustrate just how much can be packed into this space without feeling cramped. These designs prove that you don’t have to sacrifice functionality—or charm—when working with a modest footprint. Whether you’re aiming for an open, modern vibe or looking to maximize cozy nooks, the possibilities are endless.

Let’s get specific.

  • Open-concept layouts: These combine kitchen, dining, and living areas into one seamless space, with separate bedrooms and baths for privacy.
  • L-shaped plans: Add a patio or deck for bonus outdoor living space.
  • Garage conversions: These often result in quirky, charming units that don’t feel cookie-cutter.

Functional Uses of 800 Sq Ft ADUs

The true magic of an 800 sq ft ADU lies in its flexibility. These units can serve countless purposes, adapting to the specific needs of homeowners. Here are some of the most common ways they’re used:

  • Housing family (from boomerang kids to aging parents).
  • Generating rental income—51% of ADUs are long-term rentals, according to UC Berkeley.
  • Home offices or creative studios.
  • Guest spaces that let you host without sacrificing privacy.

Design Features That Work for 800 Sq Ft

The best ADU designs are a masterclass in efficiency. Every square foot counts, so it’s all about packing in functionality without making the space feel cramped. Here are some standout features that make 800 sq ft units shine:

  • Multi-functional spaces (think Murphy beds and fold-out tables).
  • Built-in storage that maximizes every nook and cranny.
  • Big windows and skylights for natural light—because no one likes a cave.
  • Energy-efficient appliances that cut costs in the long run.
  • Lofts or vertical storage for added functionality.

What is the Cost of Building an 800 Sq Ft ADU?

Let’s cut to it—how much does it cost to build an 800 sq ft ADU? Well, the range isn’t exactly pocket change: $160,000 to $320,000, give or take. The final number depends on everything from the materials you choose to how tricky your site is to prep. A hillside lot with rocky soil? That’s gonna cost you more than a flat patch of land. 

According to UC Berkeley’s Terner Center for Housing Innovation, the average cost per square foot for an ADU in California sits between $150 and $250. Multiply that by 800, and you’re looking at $120,000 to $200,000—just for construction. But that’s not the whole story, is it?

If you dig into the California ADU Owner Survey (2020)—also courtesy of Berkeley—you’ll find the median statewide construction cost lands around $150,000. That’s $250 per square foot. Not shocking, right? The math checks out, and for an 800 sq ft ADU, it adds up to roughly $200,000. But breaking down the costs? 

That’s where things get interesting.

The Nitty-Gritty Breakdown

  • Construction labor: 33% of your total bill, or about $66,000. Skilled workers don’t come cheap, especially in California.
  • Materials: 34%, so another $68,000. Whether you go for basic or bougie finishes, this chunk is unavoidable.
  • Architecture and engineering: 8%, or $16,000. You’re paying for expertise, and frankly, you don’t want to skimp here.
  • City permits: Another 8%, so again, $16,000. And trust me, dealing with permits is no walk in the park.
  • Utility connections: 5%, or $10,000. Yep, even getting the water to flow and the lights to turn on adds up.

Now, those are just averages. Real-life costs can swing wildly based on where you are and what curveballs your project throws at you. Let’s talk about the big factors.

Regional Variations—Because California Is Big

California is a beast when it comes to regional differences. The Department of Housing and Community Development has the receipts to prove it. And while the numbers don’t lie, they’ll definitely make your head spin.

Take Los Angeles, for example. The average cost for an 800 sq ft ADU might hover around $236,800—because even though LA isn’t the most expensive spot, it’s far from cheap. Now, compare that to San Francisco, where the same ADU could hit a staggering $379,200. Why the jump? Labor costs, material prices, land values—it all adds up. 

And don’t even get me started on the regulatory hoops. San Francisco loves those.

The point is, where you live in California makes a huge difference. A rural ADU in Fresno? Probably a fraction of the cost compared to something in the Bay Area. It’s all about location.

Permitting Costs: City by City

Here’s the thing about permits: they’re a headache, but they’re also non-negotiable. And for an 800 sq ft ADU, they’re gonna set you back more than a smaller unit would.

Let’s look at a few cities:

  • San Jose: Building permits might run you anywhere from $4,800 to $7,520.
  • Orange County: Fees could climb higher—think $4,800 to $11,200.
  • San Diego County: A bit more manageable, ranging from $1,600 to $8,000.

But wait, there’s more. Larger units—like, say, an 800 sq ft ADU—usually trigger additional fees:

  • School impact fees: In Los Angeles Unified, you’re looking at $3,264. (It’s $4.08 per square foot, so do the math.)
  • Parkland impact fees: These can also stack up, especially for larger units. The city wants its cut.

And that’s just scratching the surface. Always, always check your local regulations. Every city’s got its quirks.

Energy Efficiency: The Long Game

Let’s talk about saving money in the long run—because who doesn’t want lower utility bills? Adding energy-efficient features to your 800 sq ft ADU isn’t just good for the planet; it’s good for your wallet. We’re talking annual savings of $480 to $800, depending on your setup. Over a decade? That’s $4,800 to $8,000 off your original construction costs. 

Not bad, right?

Think about things like energy-efficient windows, compact appliances, or even solar panels if you’re feeling ambitious. Sure, there’s an upfront cost, but the payoff? Totally worth it.

Financing: How to Make It Work

Here’s the reality: not everyone’s sitting on a pile of cash to fund an ADU outright. But that’s okay. Financing options are more flexible than ever, and recent changes in FHA policies (October 2023, if you’re keeping track) make things even sweeter.

For instance, you might be able to finance an extra $32,000 to $64,000 through your mortgage. That’s huge—it means lower out-of-pocket costs upfront. And let’s be honest: anything that makes this process easier is a win.

Loans, refinancing, grants—there’s a whole menu of options. Do your homework, talk to your bank, and don’t forget to check for local programs that might offer additional incentives. Every dollar counts.

Pre-Approved Plans and Advantages

Pre-approved plans—sounds easy, right? Well, sort of. While many California cities roll out pre-approved ADU designs, you’ll notice fewer options for 800 sq ft units compared to their pint-sized counterparts. Why? It’s not entirely clear, but smaller units seem to hog the spotlight. That said, cities like San Jose, San Diego, and LA do have pre-approved designs for 800 sq ft ADUs.

What’s the big deal with pre-approved plans? They save time and trim costs. You skip a chunk of the back-and-forth with engineers and designers. Less red tape. Fewer headaches. It’s not a perfect solution—there’s still permitting to tackle, and those fees don’t vanish—but it takes the edge off.

If your city offers them, take advantage. It could mean cutting weeks off your timeline. And let’s face it, in California, time really is money when it comes to building anything.

FAQs About 800 Sq Ft ADU Plans

What is the maximum size allowed for an ADU?
In Diamond Bar, California (just as an example), you’re allowed one ADU per residential property, and every property gets at least 800 square feet to work with. Want something bigger? It depends on your lot size. If there’s room for it, you could go up to 1,000 square feet, but check the specifics with your city.

How much does it typically cost to build an 800 sq ft ADU?
Costs are all over the map, but $120,000 to $200,000 seems to be the usual range. That’s according to the UC Berkeley’s Terner Center for Housing Innovation, that estimates the average cost per square foot for an ADU in California sits between $150 and $250. But honestly, it depends—your city, materials, and what you’re building will all mess with those numbers.

Can I convert my garage into an 800 sq ft ADU?
Absolutely. Garage conversions are kind of the crowd favorite when it comes to ADUs. The cost? Anywhere from $80,000 to $250,000. It all depends on what you’re starting with. A clean, newer garage? Cheaper. An old, crumbling one? Not so much.

How long does it take to build an 800 sq ft ADU?
Better Place Design & Build says a garage conversion takes about 8 to 12 months from start to finish. That’s everything—planning, permits, construction. It’s not fast, but no one ever said building in California was quick.

Are there pre-approved plans available for 800 sq ft ADUs?
There sure are, at least in a lot of California cities. These pre-approved plans can speed up the permitting process, cut down on design headaches, and save you some cash. If you’re lucky, your city will have 800 sq ft options ready to go.

800 Sq Ft ADUs: Final Thoughts

Here’s the thing: 800 sq ft ADUs are like the Swiss Army knife of home projects. They’re big enough to work as a cozy one-bedroom or even a two-bedroom setup but still small enough to keep things efficient and (relatively) affordable. Whether you’re housing family, renting it out, or just adding some versatility to your property, these units are packed with potential.

Yes, costs can feel steep—$120,000 to $200,000 isn’t small change. But when you think about the long-term benefits (increased property value, rental income, and extra living space), the math starts to make sense. Pre-approved plans are a solid option if your city offers them, and garage conversions can save you a chunk of change if you’re willing to get creative.

Bottom line? 800 Sq Ft ADUs are all about flexibility. Just make sure you plan ahead, know your local regulations, and think through how you’ll use the space. Build smart, and it’s an investment you won’t regret.

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