What Is an Attached ADU?

An attached ADU is a smaller home built as part of an existing house. ADU stands for Accessory Dwelling Unit. This type of unit shares at least one wall with the main house, but it has its own kitchen, bathroom, and entrance. Some people call it an in-law suite, a guest house, or a basement apartment.

No matter the name, it works like a separate home that sits right next to the main one.

There are different ways to build an attached ADU. It might be added to the side or back of the house. It can also be placed above a garage or built by converting part of the home, such as a basement or attic. The key point is that it is connected to the main structure in some way.

Homeowners choose to build an attached ADU for several reasons. Some want a private space for parents or adult children. Others use it to make extra money by renting it out. A few folks use it as a home office, a guest room, or even a personal studio.

I’ve learned that building an attached ADU takes careful planning. The design must match the house. The construction must meet building codes. And before anything begins, zoning rules must allow it.

Different Types of Attached ADUs

Not all attached ADUs look the same. Here are a few common types:

• Basement ADU: This is one of the most common. If a house already has a finished basement, it can often be turned into a separate living space. It needs its own entrance, kitchen, and bathroom. Soundproofing is helpful to keep both spaces quiet.
• Garage Conversion: Some homeowners choose to convert an attached garage into an ADU. This works well if the garage is not being used. Insulation, plumbing, and heating need to be added.
• Side or Rear Addition: This is where a new room or wing is built onto the house. The new space is attached but acts like a small house on its own.
• Over-the-Garage Unit: In homes with an attached garage, the space above it can be used for an ADU. This option adds living space without taking away anything from the yard.

Each of these has benefits and challenges. The right choice depends on the house, the lot, the budget, and what the space will be used for.

How Attached ADUs Are Used?

Attached ADUs have many uses. Families with aging parents often build one to keep their loved ones close. This lets people stay independent while still having help nearby.

Others use it for adult children who aren’t quite ready to leave home. It gives them privacy while staying connected to family.

Some people turn attached ADUs into short-term or long-term rentals. This can bring in steady income each month. That money can help with mortgage payments, repairs, or savings.

A few folks use an attached ADU as a home office or creative space. It keeps work and home life separate without needing to drive anywhere.

Whatever the use, it helps add flexible living space to the property.

Pros of Building an Attached ADU

There are several upsides to building an attached ADU. I’ll go over some that matter most:

• Lower Cost Than Detached ADUs: Since the ADU is part of the house, some materials and systems can be shared. This keeps costs lower than building a unit from the ground up in the yard.
• Adds Property Value: Homes with extra living units can sell for more. Buyers often like the idea of rental income or flexible space.
• Multi-Generational Living: Having family members under one roof—but with private space—can make life easier for everyone.
• Rental Income: Monthly rent from an ADU can help cover costs. It gives the homeowner a new source of income without buying a second property.
• Flexible Space: The ADU can be used in different ways over time. What starts as a guest suite might become a home office or a rental later.

Challenges to Watch For

Building an attached ADU is not always simple. These are some issues to think about:

• Zoning Rules: Some cities or neighborhoods do not allow ADUs. Others have strict limits. I always recommend checking local laws before making any plans.
• Construction Costs: Even though attached ADUs cost less than detached ones, they still require money. Permits, designs, labor, and materials add up fast.
• Privacy Issues: Because it shares a wall with the main home, sound can be a problem. Good insulation and layout choices help with this.
• Parking Requirements: Some places require extra parking if you add another unit. This can be hard on small lots.
• Utility Connections: It may not be easy to set up separate water, gas, or power lines. Some homeowners choose to share systems, while others install separate meters.

Rules and Permits – Attached ADU requirements

An attached ADU in Colorado Springs can be up to 50% of the main home (max 1,250 sq. ft.), must have a private entrance, match the home’s design, include one off-street parking space, meet utility codes, and follow city permits and inspections.

Before starting an attached ADU project, it’s important to understand the city regulations, zoning rules, and design standards that apply. In Colorado Springs, the 2025 ADU ordinance set clear guidelines to make the process easier for homeowners while ensuring new units fit the character of local neighborhoods. Here are the key requirements you need to know:

• Building permits
• Health and safety checks
• Zoning approvals
• Size limits

Some places limit how big the ADU can be or where it can sit on the property. Others may require that the homeowner live on-site.

1. Size Limits

• An attached ADU can be up to 50% of the size of your primary home.
• The maximum size is 1,250 sq. ft.
• If your main home is under 1,500 sq. ft., the maximum ADU size is 750 sq. ft.

2. Design Compatibility

• The ADU must match or complement the architectural style, roof pitch, and exterior finishes of your primary residence.
• Entrances should be designed to blend naturally with the home, maintaining curb appeal.

3. Parking Requirements

• At least one additional off-street parking space is required for an ADU.
• In some cases, exceptions apply if your property is near public transit or has unique lot conditions.

4. Separate Entrance

• An attached ADU must have its own private entrance that is distinct from the main home.
• This ensures the unit functions as an independent living space while still connected to the main structure.

5. Utility Connections

• Utilities (water, sewer, and electricity) can often be shared with the main home, but must meet building code requirements for safety and efficiency.

6. Occupancy & Use

• Attached ADUs can be used for family living or long-term rentals.
• They cannot be used as short-term rentals (Airbnb/VRBO) under Colorado Springs’ current ordinance.

7. Permits & Inspections

• A full set of plans and permits must be approved by the City of Colorado Springs before construction begins.
• Required inspections include foundation (if expanded), framing, plumbing, electrical, and final occupancy approval.

Skipping permits can lead to fines or even removal of the unit later. I always suggest working with a licensed builder who knows the local process.

How Much Space Do You Need?

Most attached ADUs are between 400 and 1,200 square feet. The right size depends on the space you have and what you want to include. Even a small 500-square-foot unit can fit a living area, kitchen, bedroom, and bathroom.

Smart design makes a big difference. Things like open layouts, built-in storage, and natural light can make small spaces feel much larger.

Can You Build attached ADU on Your Property?

To find out if you can build an attached ADU, you’ll need to check:

• Your city or county’s zoning map
• Local building codes
• Lot size and layout
• HOA (if any)

Many areas across the US are becoming more open to ADUs to your property. Some places even encourage them to help with housing shortages. Still, each location has its own rules.

Final Thoughts

An attached ADU gives homeowners more choices. It can support family members, bring in rental income, or provide space to work from home. The unit stays part of the main house, but it works like a small home on its own.

Before you start, make sure to look at your local laws and think about your goals. The design should fit your home and your needs. Done right, an attached ADU can be a smart way to make the most of your property.

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