ADU Plans 101: What Homeowners Need to Know

What an ADU Is (and Why Homeowners Build Them)

An accessory dwelling unit, or ADU, is a secondary, self-contained living space located on the same lot as a primary single-family home. It typically includes its own entrance, a kitchen or kitchenette, a bathroom, and sleeping space, which is what distinguishes it from a simple bedroom or bonus room addition. ADUs go by several other names depending on where you live, including granny flats, in-law suites, backyard cottages, and secondary suites, but the underlying concept is the same: a smaller, independent home sharing a lot with a larger one.

Homeowners build ADUs for a handful of recurring reasons:

Common ADU Types

Not every ADU looks the same, and the type you choose has a direct effect on cost, construction timeline, and which local rules apply. The four general categories homeowners run into are:

The General ADU Process

Regardless of which type of ADU you're planning, the process generally follows the same four stages. Skipping the first one is the most common (and most expensive) mistake homeowners make.

1. Research Local Zoning and Rules First

Before any design work starts, you need to know what your specific city and state actually allow: whether ADUs are permitted on your lot, maximum size, height limits, setback requirements, parking requirements, owner-occupancy rules, and rental restrictions. These rules are set at the state and local level and differ significantly from one jurisdiction to the next, sometimes even between neighboring cities in the same county. For a detailed breakdown of how ADU laws compare across Texas, Florida, New York, and California, see our state-by-state ADU law comparison.

2. Design

Once you know your constraints, the design phase translates your goals (rental unit, in-law suite, home office) into floor plans and elevations that fit your lot and comply with the zoning limits you researched in step one. This is also where you decide on ADU type (detached, attached, conversion) based on your budget and site conditions.

3. Permitting

Completed drawings are submitted to your local building department for plan review. Reviewers check the plans against building code, zoning code, and often fire and energy code before issuing a permit. Review timelines are set entirely by the local jurisdiction and can vary widely, so this step should be budgeted for in both time and, in some cities, plan-review fees.

4. Construction

With a permit in hand, a licensed contractor builds from the approved drawings. Inspections happen at set points during construction (foundation, framing, electrical, plumbing, final) before the unit can be legally occupied.

Why ADU Rules Vary So Much by State and City

There is no single national ADU code. Some states have passed legislation that limits how restrictive local cities can be with ADU approvals, while others leave the decision almost entirely to individual cities and counties. That means two homes on opposite sides of a state line, or even opposite sides of a city boundary, can face completely different rules for the same type of ADU: different size caps, different owner-occupancy requirements, different parking mandates, and different rental restrictions.

Because of how much this varies, we've put the detailed legal comparison in its own guide rather than folding it into this primer. If you're building in Texas, Florida, New York, or California, read ADU Laws by State: TX, FL, NY & CA Compared for the specifics that apply to your location before you commit to a design direction.

What Apex's ADU Plans & Design Service Includes

Our ADU Plans & Design service covers the drafting and design work needed to take your ADU from concept to permit-ready construction documents, for detached units, attached additions, garage conversions, and basement or interior conversions alike. Work is done fully remote, and we serve clients nationwide with a particular focus in Texas, Florida, New York, and California.

Depending on your project, related services may also apply, including Permit-Ready Drawings & As-Builts if you need existing conditions documented, 3D Renderings & Walkthroughs to visualize the finished space before construction, and Energy-Efficient Home Design if you want the unit built with long-term utility costs in mind.

Frequently Asked Questions