There are six main types of Accessory Dwelling Units, each suited to different budgets, timelines, and property layouts. Understanding the differences helps you make the right choice for your situation.
Quick Comparison
| Type | Cost Range | Timeline | Size | Best For |
|---|---|---|---|---|
| Detached | $150K-$350K | 8-14 months | 400-1,200 sq ft | Maximum privacy & rental income |
| Attached | $100K-$250K | 6-10 months | 200-1,200 sq ft | Extending existing home |
| Garage Conversion | $60K-$180K | 4-8 months | 200-800 sq ft | Budget-friendly; existing structure |
| Basement | $50K-$150K | 3-6 months | 300-1,000 sq ft | Existing space; cold climates |
| JADU | $30K-$80K | 2-4 months | Up to 500 sq ft | Lowest cost; within existing home |
| Prefab | $80K-$250K | 4-8 months | 200-1,200 sq ft | Speed; quality control; modern design |
Detached ADU
A detached ADU is a completely standalone structure built separately from your main home, typically in the backyard. This is the most popular and most valuable type of ADU.
Pros
- Maximum privacy for both tenants and homeowners
- Highest rental income potential
- Adds the most property value
- Can be designed to any style
Cons
- Most expensive option
- Longest construction timeline
- Requires sufficient yard space
- Utility connections can add cost
Garage Conversion ADU
Converting an existing garage into a living space is one of the most cost-effective ways to create an ADU. The structure already exists, so you save on foundation and framing costs.
Pros
- Lower cost — structure already exists
- Faster build time than new construction
- No parking replacement required in California
- Existing electrical and plumbing nearby
Prefab / Modular ADU
Prefab ADUs are factory-built in a controlled environment and delivered to your property. They offer consistent quality, faster timelines, and increasingly competitive pricing.
Key Benefits
- Built in 6-12 weeks in factory (vs. months on-site)
- Consistent quality — not weather-dependent
- Modern designs with premium finishes
- Some manufacturers handle permits for you
Learn more in our detailed Prefab ADU Guide.
Explore Prefab ADU Options
Prefab ADUs can save 20-40% vs. traditional construction and cut build time in half. Compare top prefab manufacturers.
Compare Prefab ADUs →Not Sure Which Type is Right?
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