ADU Floor Plans: A DIY Guide to Drawing Your Accessory Dwelling Unit

An accessory dwelling unit — ADU for short — is a secondary living space on the same lot as a single-family home. They go by a dozen names depending on where you live: granny flat, in-law suite, backyard cottage, secondary suite. Whatever your municipality calls it, the interest has exploded over the past few years as housing costs have climbed and more states have loosened the rules that once made ADUs nearly impossible to build.

If you are thinking about adding one, drawing your own ADU floor plan is a reasonable starting point — and for many straightforward projects, it is entirely doable without hiring a drafter on day one. This guide walks through the common ADU types, how to think about small-footprint layouts, the code requirements that will shape your design, and how to put a permit-ready drawing together.

Disclaimer: ADU regulations vary dramatically from state to state and city to city, and they change frequently. Everything here is general guidance to help you understand the process — not legal or code advice. Before you finalize any plans, confirm current rules with your local planning and building departments. Do not rely on anything in this article as a substitute for that conversation.

What Is an ADU — and Which Type Fits Your Lot?

Most jurisdictions recognize a handful of distinct ADU categories, each with its own set of rules around setbacks, height, and lot coverage.

Detached ADU

A standalone structure in the backyard or side yard — the classic "backyard cottage." Because it sits on its own foundation and shares no walls with the main house, it gives the most flexibility in layout and placement. It also tends to require the most site work: new foundation, separate utility connections, and compliance with setback rules on all sides.

Attached ADU

An addition that is physically connected to the main house but has its own entrance, kitchen, and bathroom. It shares at least one wall with the primary structure. Permitting often falls under the same rules as a standard home addition, though the separate-entrance and kitchen requirements set it apart from a simple room addition. Our guide on how to plan a home addition covers the addition-side of that process in more detail.

Garage Conversion ADU

Converting an attached or detached garage into living space is one of the most cost-effective ADU options because the structure already exists. The shell — walls, roof, foundation — gets reused; you are adding insulation, windows, HVAC, and finish work. See our detailed breakdown in Garage Conversion Floor Plans for specifics on what changes when you redesign a garage as a living unit.

Junior ADU (JADU)

A junior ADU is carved out of the existing house itself — typically a basement, a large bedroom suite, or a portion of the ground floor. It is usually capped at a smaller maximum size than a full ADU (the threshold varies by jurisdiction), may share laundry with the main house, and sometimes allows a kitchenette rather than a full kitchen. The owner is often required to live on the property.

ADU Sizes and Efficient Small-Footprint Layouts

ADUs are by nature compact. Working within a small footprint does not have to mean cramped — it means being deliberate about every square foot.

Typical Size Ranges

• Studio / efficiency: 300–500 sq ft. Combined sleeping and living area, kitchenette or full kitchen along one wall, full bathroom. Fits in a one-car garage footprint or a modest backyard structure.
• One-bedroom: 500–800 sq ft. Separate bedroom, living area, kitchen, and bathroom. The most common size range for rental-focused ADUs.
• Two-bedroom: 800–1,200 sq ft. Practical for multi-generational living. Less common because it pushes against lot coverage and size limits in many jurisdictions.

Designing a Functional Studio Layout

In a studio under 450 square feet, placement of the bathroom and kitchen determines everything else. The most efficient approach is to stack or cluster the wet areas — bathroom and kitchen — along the same plumbing wall. This keeps the drainage run short, reduces the cost of rough-in plumbing, and leaves the remainder of the floor plate open for living and sleeping.

A galley kitchen along one wall is almost always more efficient than an L-shape in a small ADU — it uses less square footage and keeps the aisle clear. The bathroom should be tucked into a corner rather than placed in the middle of the plan. That single decision can free up a meaningful chunk of usable floor area in the main space.

One-Bedroom Layout Considerations

In a one-bedroom ADU, the bedroom door placement matters more than most people realize. If the bedroom opens directly into the kitchen, the plan will feel awkward to live in. Aim for the bedroom to open into a short hall or the living area instead. Keep the bathroom accessible from both the bedroom and the living area without walking through the other space.

Storage in small ADUs is almost always underestimated. Build it into the design: closets at the ends of hallways, under-stair storage if you have a loft, a pantry cabinet that doubles as a room divider, built-in shelving flanking the bed. These elements show up on the floor plan and reviewers expect to see them.

Code Requirements That Will Shape Your ADU Floor Plan

Before you draw a single wall, you need to know the rules that govern what you can build. Here are the categories that matter most — but remember, the specifics differ everywhere.

Setbacks and Lot Coverage

Every jurisdiction sets minimum distances between structures and property lines. Many states have passed ADU-friendly legislation that reduces or eliminates rear and side setbacks for ADUs compared to the rules for primary structures, but local rules vary. Lot coverage limits cap how much of your lot can be covered by structures total — adding an ADU counts against that number.

Maximum Size

Some areas cap ADU square footage at a flat number; others tie the limit to a percentage of the main house's size; others set minimum allowable sizes that municipalities cannot reduce below. Know the ceiling before you design — there is no point drawing a 900-square-foot plan if your jurisdiction caps detached ADUs at 640 square feet.

Egress Requirements

Every sleeping room needs at least one egress window or door sized to allow emergency exit. Minimum opening dimensions apply to both width and height. Ground-floor rooms can often use a door to meet egress; basement bedrooms almost always need a window well. These dimensions will drive your window placement on the plan.

Minimum Room Sizes

Building codes set minimum floor areas for habitable rooms — bedrooms, living rooms, and similar spaces. These numbers are not enormous, but they put a floor under how small a room can be. Your bathroom also needs to meet minimum clearance dimensions around the toilet, shower, and vanity.

Kitchen and Bathroom Requirements

A full ADU typically requires a complete kitchen — sink, cooktop or range, refrigerator, and adequate counter and cabinet space. A junior ADU may qualify for a kitchenette (no full-size range). The bathroom must include a toilet, lavatory, and either a tub or shower. Ventilation requirements — exhaust fans, operable windows — apply to both spaces.

Separate Entrance

Nearly every jurisdiction requires an ADU to have its own exterior entrance that does not pass through the main dwelling. Where you locate that entrance affects the whole plan — it needs to be accessible, not hidden, and often needs a landing or porch area.

Utilities

Detached ADUs typically require their own utility connections or, at minimum, a subpanel fed from the main house. Some jurisdictions require a separate water meter for ADUs; others allow shared service. Sewer capacity is often a question that comes up with the building department early. Sort out the utility strategy before you finalize the plan — it can affect where mechanical rooms, panels, and water heater space get located.

Parking

Historically, many jurisdictions required replacement parking if an ADU displaced a garage space. State-level ADU legislation in several states has eliminated or significantly reduced parking replacement requirements, particularly near transit. Check your local rules — this one has changed a lot in recent years.

Drawing Your ADU Floor Plan Step by Step

Once you know what your jurisdiction allows, you can start putting the plan together. Here is a straightforward process.

Step 1: Measure and Sketch Your Site

For a detached ADU, start with a rough site plan showing your lot boundaries, the existing house footprint, any other structures, utility lines, and the setback lines. This tells you the buildable envelope — the area where the ADU can actually go. For a garage conversion or attached ADU, measure the existing structure you are working with.

Step 2: Block Out the Program

Before drawing walls, list every space you need: bedroom, bathroom, kitchen, living area, storage, any mechanical space. Assign approximate square footages that add up to your target total. This keeps you from designing yourself into a corner where the bathroom ends up 60 square feet because everything else ate the budget.

Step 3: Draw to Scale

A floor plan submitted for a permit needs to be drawn to a recognized scale — typically 1/4 inch = 1 foot for residential floor plans. Drawing to scale forces you to confront whether your layout actually fits. It is where you find out that the queen bed you planned for the bedroom leaves no walking room once the closet and door swing are accounted for.

Easy Draft is built for exactly this stage. You draw walls, place doors and windows, and label dimensions directly in the browser — no CAD background required. When you are done, it exports a scaled PDF you can submit with your permit application. At $39 one-time, it is a reasonable tool for a project where professional drafting fees can run into the thousands.

Step 4: Add Dimensions and Notes

Every room needs labeled dimensions. Door and window sizes should be called out. If your jurisdiction requires it, note ceiling heights, egress window dimensions, and room designations. The more clearly the plan communicates the design, the fewer back-and-forth correction cycles you will have with the building department.

Step 5: Check Against Your Local Requirements

Before you submit, go through your local ADU checklist line by line. Most building departments publish one. Common items that get missed: egress window dimensions not called out, bathroom clearances not shown, no note on ceiling height, missing smoke/CO detector locations. Catching these before submission saves weeks.

Permitting an ADU: What to Expect

ADU permitting typically involves two departments: planning (or zoning) and building. They are often separate offices with separate processes, and you usually need planning approval before the building department will accept your construction plans.

Planning / Zoning Review

The planning department confirms that your proposed ADU is allowed on your lot under current zoning — the right zoning district, setbacks, lot coverage, and size. In many jurisdictions this is now an over-the-counter or ministerial approval for code-compliant ADU applications, meaning no discretionary hearing. But that varies; some areas still require neighbor notice or a hearing.

Building Permit

Once zoning is cleared, you submit construction drawings — your floor plan, plus any required elevations, site plan, and structural details — to the building department. Plan check reviews your drawings for code compliance. If corrections are needed, you revise and resubmit. When approved, you pull the permit and construction inspections begin.

Fees vary widely. Expect permit fees to run from a few hundred dollars for a simple garage conversion to several thousand for a new detached structure, depending on your jurisdiction and the project valuation.

Inspections

Most ADU projects require inspections at framing, rough plumbing, rough electrical, insulation, and final. Do not cover work before the inspector signs off — that creates problems and can require demolition to expose the work. Schedule inspections as each phase completes.

If you are also planning a room addition on the main house rather than a standalone ADU, the process overlaps significantly — our guide on how to plan a home addition covers that permitting path in detail.

For questions on when professional design help is worth the cost, see Do You Need an Architect for a Home Addition? — the same logic applies to ADU design.