400 Square Feet ADU Floor Plans
A 400 sq ft ADU floor plan is a versatile starting point — a guest house, home office, wheelchair-accessible suite for aging parents, or a rental that can house a whole family. Below are our most popular 400 sq ft ADU layouts, ready to customize and permit.
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Choose Your 400 Sq Ft Floor Plan
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How It Works
See This Plan in Your Backyard
Three steps from an idea to your ADU shown on your own lot and rendered in augmented reality - right in your backyard.
Choose from 700+ pre-designed ADU plans curated for different lot shapes, budgets, and lifestyles.
Type your address to instantly view the plans on your property map. Move and rotate for correct placement. Check the size.
See your ADU in augmented reality, right in your backyard. Walk in and around, visualize placement, and share with family.
ADU Essentials
What's Included in Our 400 Sq Ft Plans
Modern. Durable. Low emissions. Healthy. Comfortable. Money-saving. Carefree maintenance.
Open space living/dining/kitchen with natural lighting. Dual-pane glass windows for lower utility bills. Smart HVAC unit for comfort.
Your choice of cabinet doors and countertops. All electric appliances for efficiency and ease of maintenance.
High-efficiency fan for moisture control. Smooth wall finish for fast cleaning. Window for natural light and ventilation.
Solar panels for lower energy costs. Composite low-maintenance decking. Lifetime warranty shingles for durability.
View 400 Sq Ft in 3D
Floor Plan Details
This 400-square-foot ADU is designed for compact, comfortable living — perfect for individuals or couples. The layout efficiently uses every square foot, ensuring all necessary amenities are included while maintaining a modern, inviting feel.
Room by Room
Inside the 400 Sq Ft Layout
Natural light. Efficient layout. Modern design. Outdoor deck. Wood flooring. Loft storage.
The front entrance leads directly into the living room. A small wooden deck with seating provides a pleasant outdoor area for relaxation or entertaining.
The open-plan living and dining area is cozy and inviting, furnished with a sofa and small coffee table. Large windows flood the space with natural light, enhancing the sense of openness despite the compact footprint.
Adjacent to the living room, the kitchenette is compact yet well-equipped with a sink, stovetop, and storage cabinets. The seamless transition between cooking and dining areas makes meal preparation easy and enjoyable.
The bedroom provides a peaceful retreat with a comfortable queen-size bed and wardrobe storage. Windows on two walls fill the room with natural light, creating a bright, serene atmosphere.
The bathroom maximizes space while providing all necessary amenities — modern shower, toilet, and vanity with storage. The stylish fixtures and tiled finishes add a touch of luxury. A corner utility room houses the water heater with outside access.
Why 400 SqFt
Why Homeowners Build 400 Sq Ft ADUs
Most often it is because of space constraints. When you are building an accessory dwelling unit, you should try to build as big of a unit as possible — because of the economics of the price per square foot. A lot of cost goes into utilities and site work, and spreading that cost over more square feet gains you more property appreciation. Side benefit: ADUs under 750 Sq Ft are not subject to impact fees in California, saving you thousands of dollars.
Space for Aging Parents
Families want parents and in-laws close by — with independence and dignity intact. A 400 Sq Ft ADU is ideal: smaller spaces are often more comfortable for older people than sprawling homes.
Home Office with a Loft
Opt for a loft. ADUs can be as high as 16 feet — build a loft above the bedroom and bathroom and keep higher ceilings in the living room. You get a more spacious feel and a dedicated workspace above.
How Much Do 400 Sq Ft ADU Plans Cost?
Cost vs Value
Buying a floor plan alone is easy — you can find one online for $1,000 to $3,000. But a floor plan alone won't get you a permit. You need a full plan set for your city or county to process the application: site plan, floor plan, elevations, sections, roof and foundation details, engineering calculations and drawings, Will Serve letters from utilities, energy consultant report, and other documents your local jurisdiction may require (soil report, survey, etc.). A complete plan set can cost anywhere from $9,000 to $30,000.
The good news: you cannot buy a new single-family home or even a condo in most places for the price of an ADU. ADUs are less expensive to build, with lower utility connection costs and streamlined permitting. When looking at the budget, make sure you include design and permit fees, site work, utility connections, and solar.
What you get
Our Standard ADU Permit Plan Set Includes
Everything your city or county needs to process your ADU permit application.
- Title Sheet with Site Plan
- ADU Floor Plan
- Roof Plan
- Foundation Plan
- Elevations
- Building Sections
- Structural Information
- Energy Efficiency Report
Featured Plan
Garage Conversion Floor Plan
Have an old vacant car garage? Let's bring it into handy! This charming and efficient mother-in-law suite layout cleverly makes use of a single car garage — and transforms it into a mini home. This plan has a loft above the bathroom and kitchen space.
Living / Kitchen / Bedroom
All three main spaces are combined into one large open space. The living room and kitchenette are both fully open and parallel to each other. The bedroom is slightly segregated within this floor plan. We especially love the alcove seating — perfect to transform into a reading nook.
Bathroom
The bathroom is the only space that is separated from the open floor plan. It is equipped with all amenities — and also makes room for storage!
Expert Tip: Did you know that you can add 150 sq ft to the old garage even if the grandfathered garage sits closer to property lines than 4 feet? The new part will need to keep the new setbacks, but you get more space for your family or rental.
Why Build an ADU
Key ADU Benefits
Extra income and flexible housing
ADUs mean benefits for both renters and homeowners. Homeowners receive extra monthly rental income while adding housing for teachers, nurses, and other community members.
Keep family together
ADUs offer flexible, independent living space for extended family. Seniors can age in place with the care they need, while families stay together without sacrificing privacy — a more peaceful multi-generational household.
Build your equity
Enhanced property value: a well-designed ADU with thoughtful floor plans and high-quality construction delivers a strong return on investment.
More Plans
Explore More ADU Floor Plans
Our library includes a range of pre-designed floor plans — I-shaped, L-shaped, and more. Most ADUs under 400 sf feature one bedroom and one bathroom, but we can adapt plans to your requirements.
FAQs
Frequently Asked Questions About 400 Sq Ft ADUs
Absolutely. Many families find that 400 square feet is ideal for aging parents. Smaller, well-designed spaces are often more comfortable and easier to maintain for older people. You can add accessibility features like a roll-in shower, wider doorways, and grab bars. The compact layout means less to clean and heat, and everything is within easy reach. Plus, your aging parents maintain independence while staying close to family support.
Yes, especially by using a loft. ADUs can be built up to 16 feet tall, which allows you to create a loft above the bedroom and bathroom area while keeping higher ceilings in the living room. This creates a dedicated workspace above, while maintaining a spacious, open feel on the main level. The loft can also serve as additional storage or guest sleeping space.
A floor plan alone costs $1,000-$3,000. However, you'll need a complete plan set (site plan, elevations, sections, engineering, etc.) to get a permit, which ranges from $9,000-$30,000. Construction costs vary widely by location but typically run $200-$400 per square foot, plus site work, utilities, and solar. ADUs remain less expensive to build than single-family homes and often deliver strong returns on investment.
A complete permit plan set includes: title sheet with site plan, ADU floor plan, roof plan, foundation plan, elevations, building sections, structural information, and an energy efficiency report. Some jurisdictions may require additional documents like soil reports, surveys, or utility "Will Serve" letters. Our standard sets cover the essentials needed for most cities and counties.
Yes. This is a cost-effective way to create an ADU if you have an unused garage. Many of our floor plans are specifically designed for garage conversions. In CA you can add up to 150 square feet to an existing garage, even if it sits closer to property lines than standard setbacks allow. The new addition will need to meet current setback requirements, but you'll gain additional square footage.
400 Sq Ft ADU floor plans can arrive as a steel-framed kit home for faster assembly. Some of our plans are available as kit homes across the States — with site-specific engineering to account for local climate.
Start Building Your ADU Today
Start today
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