400 Square Feet ADU Floor Plans
A versatile starting point — guest house, home office, wheelchair-accessible suite for older parents, or a rental that can house a whole family.
- 20+Locations served
- 7Years of service
- 700+ADUs in 3D
- 5★Reviews
- 70+ADUs in AR
From the Largest Small Home Catalog
Choose Your 400 Sq Ft Floor Plan
Select a plan, then see it placed on your lot in seconds.
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.
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.
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.
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.
Start Building Your ADU Today
Start today
Pick a plan, drop it on your property, and walk through your ADU in augmented reality. It takes minutes — and it's free.