How to Design an ADU: A Practical Guide With Real Dimensions and Costs

ADUs come in four common forms: detached (a standalone structure, sometimes called a DADU), attached (sharing one wall with the house), a conversion (garage, basement, or attic), and a junior ADU or JADU (under 500 sq ft, carved out of the existing home).

Most jurisdictions cap ADU size. California, for example, allows detached ADUs up to 1,200 sq ft by right, and many cities set 800 to 1,000 sq ft limits. Typical builds land at 400 to 500 sq ft for a studio, 600 to 750 for a one-bedroom, and 800 to 1,000 for a two-bedroom.

Setbacks matter before you draw anything. California ADU law requires just 4-foot side and rear setbacks, and single-story height is commonly capped around 16 to 18 feet, higher near transit. Check your city's zoning code first — these numbers define the outer box your design has to fit inside.

The single biggest cost saver in ADU design is stacking your 'wet' rooms. Put the kitchen and bathroom on one shared plumbing wall so a single run of supply and drain lines serves both. If the ADU sits near the main house, orient that wall toward the existing sewer lateral to shorten the connection.

For a compact unit, an open-plan living-kitchen-dining zone reads far larger than walled-off rooms. A 450 sq ft studio can comfortably hold a 12x14 living area, a 9-foot kitchenette along one wall, a 5x8 bathroom, and a sleeping alcove.

In a one-bedroom, a primary bedroom of 11x12 to 12x14 fits a queen bed with walk-around space on three sides. Keep the bathroom reachable from the hallway or living space — not only through the bedroom — so renters and guests aren't routed through a private room.

Habitable rooms need a ceiling height of at least 7 feet (7 feet 6 inches in some jurisdictions). Bedrooms must be a minimum of 70 sq ft with no dimension under 7 feet, and every sleeping room needs an egress window: a clear opening of at least 5.7 sq ft, minimum 20 inches wide and 24 inches tall, with the sill no higher than 44 inches off the floor.

Interior doors should give at least 32 inches of clear width if you want the unit to be accessible, and hallways at least 36 inches. In the kitchen, follow the work triangle — the total distance between sink, stove, and refrigerator should sum to 13 to 26 feet, with 42 to 48 inches of clearance between opposing counters.

A full bathroom fits in 5x8 (40 sq ft); a three-quarter bath with a shower instead of a tub works in 5x7. These are minimums — a few extra inches in each room makes a small ADU feel dramatically less cramped.

ADU construction typically runs $150 to $400 per square foot, depending on region, finishes, and site conditions. A garage conversion is the cheapest path at roughly $100 to $200 per square foot, because the foundation, walls, and roof already exist.

A new detached ADU of 600 to 800 sq ft commonly lands between $200,000 and $400,000 all-in, including design, permits, utilities, and finishes. Prefab and panelized units can trim time and sometimes cost, but site work — foundation, utility trenching, and the sewer tie-in — is rarely included in the sticker price.

Budget utilities separately. Running new water, sewer, and electrical service, or adding a sub-panel off the main house, often adds $5,000 to $25,000 depending on the distance to existing lines.

If the ADU is for aging parents or future accessibility, design it single-story with a no-step entry, 36-inch doorways, blocking behind the bathroom walls for future grab bars, and a curbless shower. These choices cost little during framing and a great deal to retrofit later.

If you're renting it out, prioritize a private entrance separated from your own, in-unit laundry (a stacked washer-dryer needs about a 30x30-inch closet), and real storage. Renters judge small units on closet space almost as much as on square footage.

Before you pay an architect or order a prefab, sketch the layout to scale so you can see whether your furniture and clearances actually fit. Our free floor plan generator draws plans with real dimensions — walls, doors, and rooms measured to the inch — so a 5x8 bathroom is genuinely 5x8, not a rough box.

For a head start, browse pre-built layouts on the ADU floor plans page at /adu-floor-plans, then resize rooms to match your lot's setback box and your budget. Seeing accurate dimensions early is the fastest way to catch a plan that looks fine on paper but won't pass code or fit your yard.