How Much Does an ADU Cost?

As Accessory Dwelling Units (ADUs) gain popularity and land use restrictions ease, homeowners with ample backyard space are turning to these compact, fully equipped homes for family space or rental income. But how much does it cost to build an ADU?

We break down the factors affecting the cost of building an ADU using a prefab kit, from construction expenses to hidden fees, offering clarity for potential builders.

Understanding the Cost of Building an ADU

Building an ADU is an investment that pays off in flexibility, functionality, and potential income. By understanding the costs and planning carefully, you can create a space that meets your needs while staying within budget.

While ADUs come in various sizes and designs, most jurisdictions impose size limitations. Many ADUs average 600 to 1,200 square feet. Some areas set limits based on the primary residence. California limits the size to not more than 50% of the primary dwelling.

Several factors influence the overall cost of building an ADU.

Going From Kit to Completion

You get to make many of the decisions about the ADU beyond the model you choose.

Cost of the Kit

ADU kits provide a convenient option for homeowners, with prices ranging from $40,000 to $120,000 depending on size and features. Shipping costs are extra and vary by location. For instance, ADU costs in California might be higher due to increased shipping fees.

Foundation Pricing

Choosing a foundation type is a necessary step. Common home foundations include slab-on-grade, crawl spaces, or basements. Depending on the home's square footage, they can range from $4,000 to $40,000 or more.

Soil conditions, site preparation, and grading can significantly affect this cost.

A concrete slab foundation reduces cost.

Diy work crew celebrates the completion of the wall and roof panels. — You plus a few friends and family members can assemble the wall panels and roof, reducing costs.

Builder vs. DIY Construction Costs

Hiring a builder or contractor increases the overall cost, with labor accounting for 40% to 50% of the budget.

DIY construction can save money. Based on your skills or those of family or friends, prefab kit ADUs, like those from Might Small Homes, can offer a more affordable route to building your ADU.

You can reduce labor costs by taking on the wall and roof assembly, basic installations, and performing finishing touches.

Be realistic about your time commitment and expertise.

Exterior Finishing

Overhead, the roofing material—asphalt shingles, metal, or tile—adds durability and style for $5,000 to $15,000.

The siding— whether, vinyl, wood, brick, or fiber cement among the most common options—completes the exterior with a price range of $5,000 to $12,000. Each element plays a part in protecting and transforming your ADU.

New on page alt - cement veneer exterior offers fire resistance for this California home — The cement veneer finish offers fire resistance for this California home.

New for this article instance: natural pine ceiling hardwood floors add rustic look. — Optional features like hardwood flooring and natural pine ceiling create a modern look at additional costs.

Interior Finishing

Interior finishing makes your ADU welcoming, usable, livable, and visually appealing.

In the kitchen, recessed lighting highlights sleek cabinets and countertops, representing a $10,000 to $20,000 investment. The HVAC system, a $2,000 to $5,000 expense, ensures every room feels comfortable year-round.

Plumbing and electrical systems, though hidden, represent costs totaling between $8,000 to $15,000 combined.

Managing Costs - Nice to Have vs. Must Have

Similar to buying a new car, prioritizing essential options and passing on the extras can keep costs in check. Regardless of the size of the home, each Mighty Small Homes model includes a custom floor plan and the lumber needed for interior framing. Also, most kits can accommodate a loft at no additional charge.

Low-End Budget

If working with a more limited budget, you must prioritize affordability while ensuring your ADU meets standards and building codes. At this price point, your ADU will likely be a smaller, no-frills structure focused on functionality over luxury.

Basic ADU Kit

If not required by building codes, choosing standard wall and roof panels, which can still reduce utility costs by nearly 60%, helps keep expenses in check. However, if noise or temperature extremes are a concern, upgrading to optional thicker panels may be a consideration. Your kit cost would increase by about 18% with these options. Foundation
A simple slab-on-grade foundation is the most budget-friendly option, requiring minimal excavation and site preparation. However, costs may increase slightly if the terrain needs additional grading.

Roofing & Siding

Basic asphalt shingles for the roof and vinyl siding for the exterior provide durability at an affordable price. These materials are cost-effective but may require more maintenance over time.

Asphalt shingles, available in a variety of colors and shades, are one of the most affordable roofing options.

Polished concrete slab flooring can reduce interior finishing costs.

Interior Finishes

Lower-cost interior finishes typically include basic flooring options like vinyl or low-grade carpet and simpler trim and molding made from MDF or plywood.

These cost-effective choices may lack the aesthetic appeal and durability of higher-end materials. Using budget-friendly materials and finishes can save an average of 20-40% over high-end materials.

Kitchen & Bathroom

Low cost mdf particle board cabinet panels — Particle board with laminate finish can reduce kitchen cabinet costs

Cutting costs in the kitchen and bathroom often involves opting for simpler materials, like particle board cabinetry with laminate-facing and similar materials for countertops. Basic plumbing fixtures save money but lack the durability of premium options. For a 1,000-square-foot home, budget about $8,000-$10,000.

HVAC & Utilities

Expect a minimal heating and cooling setup, such as a window AC unit or a budget-friendly split system. A standard water heater and basic electrical work will cover essential power needs, but you may need to limit the number of outlets or lighting fixtures. Choosing a lower-efficiency HVAC and water heating system reduces cost. A 12,000 BTU single-zone mini-split system may cost about $4,000. Costs increase substantially for a higher-efficiency, multi-zone system.

This budget range allows you to build a livable, functional ADU, but it will be a more straightforward space with fewer upgrades or customization options.

Mr cool components of diy hvac single mini split — MRCOOL DIY® makes mounting and installation easier with a patented system. That can save you money.

High-End Budget

For those with a larger budget, an ADU can become a premium, customized space with luxury finishes, advanced technology, and high energy efficiency.

ADU Kit or Custom Build

At this price range, you can opt for a larger ADU between 1,000 to 1,200 square feet with custom features like decks and covered porches. Thicker roof and wall panels, which add energy efficiency, can be added.

Local building codes may require thicker roof panels for snow loads. Additionally, non-standard window and door openings can be included.

Foundations

In progress homestead with walkout basement — This home is built on a walkout basement foundation, adding additional livable square footage.

A reinforced foundation with thorough site preparation—such as clearing rocks, trees, and grading—ensures long-term stability. Depending on soil conditions and preferences, options include a raised foundation, crawl space, or basement.

While more expensive, a basement can double the livable space. Costs for a larger footprint may range from $20,000 to $40,000, including waterproofing and drainage.