If you are trying to estimate the ADU construction cost and keep coming across those tempting ads claiming you can build an ADU for as little as $10k—stop right there. Take a moment before getting too excited. Those numbers are not real. The truth is, the cost of building an ADU is much more complex, and there are many hidden expenses that homeowners usually overlook at the beginning.
Realistically, an ADU build cost can range anywhere from $65k to $300k, depending on the materials, construction type, and workforce you choose. That’s a wide range, and it can feel overwhelming, but don’t worry—we’ll break it all down for you. In this guide, we’ll go through the main cost categories, the hidden expenses that often appear during construction, and some ways to reduce your ADU cost without cutting corners or sacrificing quality.
Main Categories for Accessory Dwelling Unit Costs
In short, the accessory dwelling unit cost can be divided into three main categories:
- Soft costs: design, engineering, and permits (~10%)
- Hard construction costs: foundation, structure, framing, MEP systems, finishes, etc. (~80%)
- Unexpected costs: delays, hidden conditions, mistakes, or changes (~10%)
The balance between these categories is critical. By making poor or rushed decisions in any part of the process, your overall ADU build cost can increase exponentially.
For example, if you plan to build an ADU for $150k, your soft costs will be around $15k at most. But if you choose inexperienced engineers, submit documents incorrectly, or face permit rejections, your costs can climb much higher. The good news? You can reduce these risks by working with experienced professionals, using a clear permit submission strategy, and minimizing the probability of rejection. That alone can save thousands
Construction Costs for Different Types of ADUs
When it comes to building an ADU unit, not all are equal. The type of ADU you decide to build has a huge impact on the ADU construction cost. Let’s look at the most common types and their differences:
A) Detached ADUs Costs
Detached ADUs usually carry the highest ADU cost. Why? Because they are independent units—standalone buildings often constructed in the backyard or a separate section of a multi-family lot. They require a brand-new foundation, separate utilities, and complete exterior work.
Everything is built from scratch, which makes detached ADUs costs the highest among all ADU types. While they give you the most privacy and flexibility, they also come with the largest price tag.
B) Attached ADU Costs
An attached ADU usually costs less than a detached one. Since it shares at least one wall with the main residence, you save on part of the foundation and exterior work. In some cases, it can even share existing plumbing, electrical, or HVAC systems, which reduces expenses further.
The cost of building an ADU as an attached unit is still significant, but the shared structure and utilities bring the price below the detached category.
C) Garage Conversion Cost
Garage conversions are another popular choice. At first glance, they seem affordable because you’re working with an existing structure. You can reuse the foundation, some walls, and even parts of the roof. However, the reality is that garages are not designed as habitable spaces.
To turn a garage into a livable ADU, you often need to:
- Add insulation and drywall
- Retrofit or upgrade structural elements
- Sometimes lift the roof to meet height requirements
- Install full plumbing and electrical systems
That’s why the garage conversion cost usually falls into the third place in comparison with detached and attached ADUs. It’s lower than building from scratch, but still not as low as many homeowners expect at the start.
D) Basement or Interior Conversion Costs
Basement or interior conversions generally represent the lowest ADU build cost. That’s because the foundation, roof, and exterior walls already exist. In some cases, the space has even been used as a living area before.
But beware of hidden costs. Excavation, waterproofing, and creating proper egress solutions can add significantly to the budget. You might even discover halfway through that your entire plumbing system needs to be replaced. These kinds of surprises often create unexpected ADU construction costs that weren’t obvious in the beginning.
In general, the ranking is clear:
- Detached ADUs costs = highest
- Basement/interior conversions = lowest
- Garage conversions = somewhere in the middle
What Factors Affect ADU Construction Cost?
There are many factors influencing the final ADU cost. Some are completely out of your control, like city fees. Others are under your control, like the level of finishes you choose. But then there are those tricky ones that fall in between—you can’t control their existence, but you can control how often they occur or how much they end up costing you.
Here are a few of the biggest factors:
- Permit and Engineering Costs
You might not think of permits as a major part of your ADU build cost, but they are. In fact, permit acquisition alone can account for 10% of your total accessory dwelling unit cost. If you’re not familiar with the process, this becomes a hidden trap. Multiple submissions, extra review fees, and document re-uploads can force you into paying far more than you expected.
- Design & Engineering Services
Some homeowners try to save money by buying pre-designed or pre-engineered plans. But every site is different, and every city has its own codes. What looks cheap upfront usually becomes expensive later, because each “small adjustment” costs more and more. Many companies that sell generic plans don’t offer proper support during the permit stage either. This is especially painful when dealing with detached ADUs costs, since cities usually have stricter requirements for them.
- Project Management & Supervision
This is where hidden costs really explode. Without proper project management, your ADU construction cost will climb due to wasted materials, poor scheduling, and inefficient labor. A lack of supervision can turn a $150k project into a $200k one before you realize what happened. A professional project manager doesn’t just keep the project moving—they save you money, time, and stress.
Hidden Costs of Building an ADU
Beyond the main categories, there are other expenses that homeowners don’t usually see coming:
- Utility upgrades: Your electrical panel or sewer line may not have enough capacity.
- Site preparation: Grading, soil issues, or tree removal can add unexpected costs.
- Impact fees: Some cities still charge them for ADUs.
- Material price fluctuations: Lumber, concrete, and steel costs change quickly.
Each of these can push your ADU build cost above your original budget if you don’t prepare for them.
Conclusion
To wrap it up, the ADU construction cost is something you can’t control completely—but you can definitely manage. By making professional and well-informed choices, you can keep your ADU cost within an optimal range that suits your budget.
You can reduce the garage conversion cost, cut down the soft costs, and avoid repeating mistakes that multiply your expenses. Or, if you’re careless, you can double your accessory dwelling unit cost without even noticing.
The difference comes down to choices. Smart decisions about permits, engineering, project management, and material selection can save you thousands. Building an ADU is not only about spending money—it’s about investing wisely in your property and your future.
In the end, your ADU can either become a painful financial burden, or a sweet and valuable addition to your home. The choice is yours.