What Is an ADU? A Homeowner's Guide for Nashville and Indianapolis

If you've spent any time browsing home renovation ideas lately, you've probably run into the term ADU. It stands for accessory dwelling unit. Housing costs keep climbing, families are rethinking what "home" looks like, and homeowners want more flexibility from the property they already own.

An ADU gives you that flexibility. It's a smaller, independent living space built on the same lot as your main home. Think of it as a second home in miniature, complete with its own entrance, kitchen, and bathroom.

This guide breaks down what counts as an ADU, why so many homeowners are building one, and what to think through before you start.

What Counts as an ADU?

Not every backyard structure qualifies as an accessory dwelling unit. The key requirement is independence. An ADU needs its own kitchen, bathroom, and entrance, separate from the main house.

Attached vs. Detached vs. Converted

ADUs generally fall into three categories:

• Attached ADUs share a wall with the main house, often as an addition or a converted section of the home.

• Detached ADUs (sometimes called DADUs) stand completely separate from the main house, usually toward the back of the lot.

• Converted ADUs repurpose existing space, like a basement or a garage, into a livable unit.

Each type comes with its own cost, timeline, and design considerations. A converted basement might be the fastest and most budget-friendly option. A detached unit gives you the most privacy but usually costs more to build from the ground up.

Where DADUs Fit In

A DADU is simply a detached ADU. It's a standalone structure, fully separate from your main home. We cover DADUs in more depth in a separate guide, but it's worth knowing the term now since you'll see it used interchangeably with "detached ADU" in permitting documents and zoning codes.

Why Homeowners Are Building ADUs

The motivations behind an ADU project tend to fall into a few common categories.

Rental Income Potential

A well-designed ADU can generate steady rental income, whether you're renting long-term or hosting short-term guests. For many homeowners, this income helps offset the cost of construction over time.

Housing for Aging Parents or Adult Children

Multigenerational living has become far more common, and an ADU offers a practical solution. Aging parents get their own space while staying close to family. Adult children can save on rent while working toward independence. Everyone gets privacy without sacrificing proximity.

Increasing Property Value

A thoughtfully built ADU adds usable square footage to your property, which can meaningfully increase resale value. Buyers increasingly see ADUs as a selling point, not just an add-on.

Zoning and Permitting Basics

This is the part of the process that trips up the most homeowners, mainly because the rules vary so much depending on where you live.

How Zoning Varies by City and County

Zoning codes for ADUs differ significantly between Nashville and Indianapolis, and even between neighborhoods within the same city. Some areas allow ADUs by right, meaning you don't need special approval as long as you meet the requirements. Others require a variance or conditional use permit. Before you get attached to a design, it's worth confirming exactly what your specific lot allows.

Common Restrictions

While the details vary, a few restrictions show up often:

• Maximum size, usually expressed as a percentage of the main home's square footage

• Owner-occupancy requirements, meaning you may need to live in either the main house or the ADU

• Parking requirements, which can affect where you're able to place the unit on your lot

None of these restrictions are dealbreakers, but they do shape what's actually possible on your property.

Cost Factors to Understand Early

ADU costs vary widely, and getting a realistic number early saves a lot of frustration later.

Site Prep, Utilities, and Permitting Fees

Before construction even starts, you're paying for site preparation, utility connections, and permitting fees. These costs add up faster than most homeowners expect, especially if your lot needs grading or your utility lines need to be extended.

What Drives Cost Up or Down

A few factors have an outsized impact on your final number:

• Attached units typically cost less than detached ones, since they share existing walls and utility lines

• Higher-end finishes and custom layouts increase cost quickly

• Site conditions like slope, soil type, and access all affect labor and material costs

Getting a clear cost picture early lets you make informed decisions about scope before you're too far into the process to adjust.

Design Considerations

A good ADU doesn't feel like an afterthought. It feels like a genuine extension of your property.

Matching the Main Home's Architecture

Consistent rooflines, materials, and trim details help an ADU feel like it belongs, rather than looking like a mismatched add-on. This matters both for how the property feels day to day and for how it appraises down the line.

Making the Space Feel Full-Sized

Smart layout choices, like vaulted ceilings, larger windows, and open floor plans, can make even a modest square footage feel spacious. Small doesn't have to mean cramped.

Working With an Architect vs. DIY or Contractor-Only

It's tempting to skip design and go straight to a contractor, especially on a smaller structure. In practice, that approach often costs more in the long run.

An architect who understands local zoning can catch issues before they become expensive problems. Permitting rework, redesigns mid-construction, and layout changes after the fact all cost more time and money than getting it right the first time. Good design isn't an extra step. It's what keeps the rest of the project on track.

If you're weighing your options for who to bring in, our ADU builder guide breaks down what to look for when choosing the right team for your project.

Getting Started

An ADU is a meaningful investment, and the right approach depends on your goals, your lot, and your local zoning rules. Before you commit to a design, it's worth taking the time to understand what's actually possible on your property.

Ready to see what an ADU could look like on your lot? Get a Free Quote from Fabl Design and let's talk through your options.