If you’ve started exploring the idea of building a detached ADU, you’ve probably noticed something quickly: construction pricing varies—a lot. Between site conditions, design choices, utility requirements, and finish selections, no two projects look exactly the same.
At Inspired ADUs, we spend a lot of time helping homeowners understand what’s actually driving those numbers, and why Bay Area ADUs tend to fall into predictable cost ranges. This guide breaks down the core components of ADU construction costs and explains how each category contributes to the overall budget.
For most detached ADUs in the Bay Area, construction costs typically range from $500–$600 per square foot, with higher costs tied to site complexity and finish choices.
Typical Bay Area ADU Cost Ranges
For most detached ADUs in the Bay Area (typically 1–2 bedrooms), construction costs generally land between $500 and $600 per square foot. Projects can reach $650+/sf when there are complexities such as challenging sitework, premium window packages, or more advanced mechanical systems.
When costs exceed $700+/sf, they almost always involve one or more of the following:
These outliers aren’t the norm—but they do happen, and understanding the categories below will help you see why.
High-Level Cost Structure
While every general contractor organizes their cost categories a bit differently, most ADU budgets are built from the same core components. Below is a representative ROM (rough order of magnitude) breakdown. The percentages of project budget for these categories are rough guideposts and can vary significantly, depending on how a builder categorizes or the cost of his subcontractors for various trades.
Each category below includes a description and examples to help you understand how your own property and design choices influence final pricing.
1. General Conditions
This category covers the infrastructure needed to run a safe, compliant, and organized construction site for the life of the project.
Examples include:
These costs scale with project duration—longer builds require more oversight and more site support.
2. Sitework, Demolition & Utilities
Sitework is one of the biggest variables in ADU construction. It includes everything needed to prepare the property before vertical construction begins.
Examples include:
Factors like narrow access, steep slopes, clay soils, and long utility runs can significantly increase costs in this category.
3. Foundations
This includes all structural concrete work required to support the ADU.
Examples include:
Foundation costs depend heavily on soils reports, engineering requirements, and site constraints.
4. Framing & Roof Structure
This is the structural skeleton of your ADU, from floor framing all the way to the roof.
Examples include:
The complexity of your roofline—vaults, hips, dormers, or higher pitches—can increase cost and time.
5. Building Envelope
The envelope is what protects the interior from the elements and defines the exterior aesthetic.
Examples include:
Window choices are major cost drivers—vinyl is economical, while fiberglass or aluminum-clad units can increase costs significantly.
6. Interiors
Interior finishes are where homeowners have the most control over cost. This category impacts the final look and feel of the ADU.
Examples include:
Choosing durable but cost-conscious finishes can create meaningful savings without compromising quality.
7. MEP (Mechanical, Electrical & Plumbing)
This is the functional backbone of your ADU—the systems that make the home comfortable and livable.
Examples include:
MEP costs increase when service upgrades are required, when HVAC systems need multiple heads, or when plumbing layouts become complicated.
8. Exterior Final Work
Once construction is complete, this category ties the new unit into the surrounding property.
Examples include:
Some homeowners opt for minimal exterior work; others choose to fully re-landscape the impacted area.
Understanding General Contractor Overhead & Profit (OHP)
Most general contractors include overhead and profit (OHP) in their pricing, typically ranging from 12%–18% of the construction cost. This covers business operations, insurance, supervision, and the profit required to run a stable, reliable company.
Contractors present OHP in two common ways:
Neither approach is better; they’re simply different estimating styles. What matters is understanding how your contractor structures pricing so you can make fair, apples-to-apples comparisons across bids.
Bringing It All Together
Understanding where your dollars go is one of the most empowering parts of planning an ADU. While every property is unique, the overall structure of the budget remains consistent across projects.
At Inspired ADUs, we aim to provide transparency at every step—helping homeowners make informed decisions about their design, materials, and scope so their ADU aligns with their budget and their goals.
If you’re considering an ADU and want to explore realistic cost scenarios based on your property, we’re always happy to help walk through the numbers, book your call today!
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