How Much Does It Cost to Build an ADU?

ADUs are accessory dwelling units located on the same lot as the primary residence. These can be of different types of buildings, such as detached structures, extensions, garage or basement conversions. ADUs continue to grow in popularity in the United States, offering owners the opportunity to add more living space, rental income, or housing for family members. People build them for many reasons, including moving aging parents, generating some rental income, or having a personal workspace. But how much does it cost to build an ADU? Let’s figure it out.

Types of ADUs and Their Costs

Detached ADU

A detached ADU is a structure separated from the primary building on a property. Most small cottages or tiny houses are included in the category. Costs vary a lot—from $50,000 to $500,000. On the whole, the median price is $100,000-$300,000. Again, this depends on size, materials, and location. Detached ADUs are more isolated and private, meaning they provide independence; no wonder rental units are regularly a good reason for them. These units may also be used as home-based offices, cabanas for guests, or to earn extra income through Airbnb. The ADU price range for detached units is broader due to varying adu costs and adu construction costs.

Attached ADU

An attached ADU is built connecting to the main house, which could be an addition to either side or the back of the home. It generally costs less than a detached ADU, although in the range of between $50,000 to $200,000 or thereabouts, of course, many factors go into determining an ADU price precisely. Attached ADUs usually have relatively lower construction costs as their infrastructure is partly shared with the infrastructure of the main house, like walls and facilities in place. This type benefits those family members who want to live close but, at the same time, be independent.

Garage Conversion

Garage conversions for an ADU are common. The new construction is relatively inexpensive. The cost would vary between $40,000 and $100,000, depending on the condition of the current garage and how much renovation work should be done. Garage conversions can be perfect for using some existing space without expanding the square feet of your property. This kind of ADU works perfectly to efficiently create a living space with minimal environmental impact, as it makes use of existing structures.

Basement Conversion

A basement conversion can also be a cost-effective way to create an ADU. Costs generally range from $30,000 to $90,000. Since A DU is an accessory dwelling unit, in most cases, it will be pretty cheap to create, as most of the structure is already in place, although the work to render it livable might be pretty considerable. The most common improvements are waterproofing egress windows and insulation to meet building codes and make the space comfortable. Converting a basement to an ADU can also add to the overall value of your home. How much to build an ADU like this can be influenced by the extent of necessary renovations.

Fixed and Variable Costs and How They Affect the Total Price

In building an ADU, one has to be proficient in distinguishing fixed from variable costs. Fixed costs remain constant regardless of whether your project is small or big, simple or complex. On the other hand, variable costs will vary according to certain factors: materials and labor. Below is a table for quick illustration:

Cost Type



Impact on Total Price

Fixed Costs

Costs that remain constant regardless of changes

Permits, Design Fees, Survey Costs

Provides a baseline for the project

Variable Costs

Costs that change based on project specifics

Materials, Labor, Landscaping, Interior Finishes

Can significantly increase or decrease

This way, knowing these costs will help you plan and budget in a more quality manner. For example, while there is nothing you can do to change fixed costs such as permits and design fees, variable costs can be managed by selecting less expensive materials or doing part of the work yourself.

ADU Cost Calculator – Main Factors Affecting ADU Costs

Size of the ADU

The size of the ADU greatly impacts the cost. Larger units demand more materials and labor, thereby increasing the price. An ADU with 600 square feet will cost more than one that has 400 square feet. Size must, therefore, be carefully planned to adjust the cost with maximum functionality visibly. Larger ADUs would yield more living space but increase construction and maintenance costs correspondingly.


The choice of materials used forms a large part of the whole cost. High-end finishes and materials raise the price. Not-so-expensive materials may help cut down on costs. For example, luxury flooring or countertops increase expenses, while more standard alternatives will help save on costs. 

Furthermore, the available types of materials vary by region, affecting their costs due to transportation costs. Sustainable materials, although sometimes costly upfront, can save money in the long run and contribute much towards environmental conservation.

Design and Architectural Features

The more complex the design and individual features an architectural design possesses, the more one must pay for it. Simpler designs typically cost significantly less. Bespoke designs require more specialized labor and materials, which tend to increase the cost of construction. However, a well-designed ADU can add value to your property. 

Architectural features, such as vaulted ceilings, lots of big windows, or some high-end custom cabinetry, will not only look beautiful but will serve better in functionality and, at the same time increase the cost of your budget.


ADU construction costs vary by region. The area with high labor costs and material input is, of course, more expensive. Local building codes will also affect permit fees. Urban areas with strict zoning laws may have higher costs than rural areas with more lenient ones. Also, since the general cost of living in your region is higher, labor rates and the availability of construction services may be more aggressive.

Utility Connections

Connecting to utilities such as water, electricity, and sewer can add to the cost. The further the ADU is from these connections, the greater the price of the task will be. An accessory dwelling unit constructed in the back of a large property can quickly require the extension of utility lines, adding a not-so-insubstantial expense to the job. Upgrading existing systems to handle access to the ADU can be costly if your existing utilities are outdated.

Hidden and Additional Costs

Planning and Design

Hiring an architect or designer can add to the cost. In most cases, experts of this nature will make your ADU functional and compliant with local codes. You should expect them to charge you anything between $2,000 and $10,000 for their services. 

While this is another cost but professional Design Services can save you money in design errors or omissions, which may later prove to be costly. They guide one through the zones on compliance and authorized permits.

Permits and Legal Fees

Building an ADU could be numerous permits, from a building permit, an electrical permit, or a plumbing permit. Permit costs could range from between $1,000 to $8,000. 

It’s important to check with your local building department to understand the specific permits required in your area and their associated costs. Also, a lawyer may be needed to sort out zoning variances or disputes over property lines.

Landscaping and Exterior Work

Landscaping an ADU will add even more to its final cost. You likely must also commit to other exterior improvements, like a driveway or deck. These expenses might add $1,000 to $10,000 for a driveway or patio. Landscaping will beautify your ADU and provide additional outdoor living space. Low maintenance costs should be considered, as well as drought-resistant plants, to keep long-term maintenance costs down.

Furniture and Interior Finishing

Finally, do not forget to furnish your ADU, including anything from appliances to furniture. Other additions may add much to the budget for finishing, which provides for interior elements like flooring, paint, and lighting. They could go for as much as $5,000 to $20,000. 

High-quality finishes and appliances make your ADU more attractive to potential renters or guests. Durable materials that are easy to clean also result in reduced maintenance and replacement fees in the future.

Savings and Financial Assistance

Grants and Subsidies

There are grants and subsidies available to help cover the cost of building an ADU. Check with local housing authorities for available programs. Some cities offer financial incentives to encourage the construction of ADUs to address housing shortages. These programs can significantly offset initial costs, making ADU construction more affordable.  Understanding how ADU value is calculated can also help you see the potential financial benefits of these programs.

Loans and Financing

If estimated costs are over planned resources, many banks and credit unions today offer ADU-construction-matched loans for such particular projects. These loans stretch the costs over time. Familiar sources of financing are home equity lines of credit or loans.

It is important to shop at different lenders and compare the terms of these loans. Some lenders have special rates available for green or energy-efficient construction; these may further drive costs down.

Cost-Saving Tips

To save money, consider using cost-effective materials. Keep the design simple and avoid unnecessary features. Doing some of the work yourself can also reduce costs. For example, you might handle painting or landscaping to save on labor expenses. 

Additionally, purchasing materials in bulk or during sales can help lower costs. Reusing or repurposing materials from the main house can also be a cost-effective and environmentally friendly option.


Building an ADU is an enormous investment. The costs are as varied as the dimensions, materials, design, and location. Planning is essential; considerations need to be given to every expense. Working with professionals can help ensure your ADU meets all local regulations and is built to last. Whether you’re creating extra space to live, rent, or host your family, an ADU can pay off with value to your property. Good preparation with budgeting gives you the most out of your investment and a brilliant functional living space.

Frequently Asked Questions (FAQ)

How the selection of a contractor could affect the final building cost of ADU

The choice of contractor highly influences the cost. Though at a bit higher price, experienced contractors save dollars, in the long run, on quality and efficient work, thus enabling money to be saved for future repairs and possible alterations. 

Do these ADUs cost differently to build between an urban or rural setting?

These can vary greatly. The labor and materials cost more in urban areas, and the rules and regulations are tight, which might cause more expensive permits. Rural areas indeed come cheap, but one might incur some expenses related to transport, materials, and utility connections.

Are there any tax implications or benefits related to building an ADU that might affect its overall costs?

The addition of an ADU may increase property value and property taxation. Conversely, an ADU may generate a rental income that refers to fewer tax obligations. To weigh such implications firsthand, one has to consult a tax advisor.

How can I estimate the long-term return on constructing an ADU?

To estimate ROI, one needs to consider construction costs, potential rental income vis-à-vis property value appreciation, and ongoing maintenance costs. A comparison of these factors can point out the financial benefits of constructing an ADU.