Prefab ADU vs Custom-Built ADU: Which is Cheaper in San Diego?
Comparing prefab ADU costs ($100K-$200K) vs custom-built ADUs ($150K-$350K). Pros, cons, and which option is right for your San Diego property.
When planning an ADU in San Diego, one of the biggest decisions is whether to go with a prefab (modular) ADU or a custom-built (stick-built) ADU. Here's an honest comparison.
Prefab ADU Overview
Prefab ADUs are manufactured off-site and delivered to your property. They come in standardized designs and sizes.
Prefab ADU Cost Range
- Unit cost: $80,000-$150,000
- Site work and foundation: $30,000-$50,000
- Utility connections: $15,000-$30,000
- Permits and fees: $10,000-$20,000
- Total installed: $135,000-$250,000
Prefab ADU Pros
- Faster construction (unit built while permits process)
- Predictable pricing
- Quality controlled factory environment
- Less disruption during on-site installation
Prefab ADU Cons
- Limited customization options
- Standard sizes may not maximize your lot
- Delivery access issues on narrow lots or hillsides
- Hidden costs for site preparation and connections
- May look "cookie-cutter" compared to main home
Custom-Built ADU Overview
Custom ADUs are built on-site from the ground up, designed specifically for your property and preferences.
Custom ADU Cost Range
- Design and permits: $15,000-$30,000
- Construction: $300-$450/sq ft
- 500 sq ft total: $165,000-$250,000
- 800 sq ft total: $255,000-$380,000
- 1,200 sq ft total: $375,000-$560,000
Custom ADU Pros
- Complete design flexibility
- Matches your home's architecture
- Maximizes your specific lot layout
- Works on challenging sites (slopes, tight access)
- Higher quality potential with premium materials
- Often better long-term value and rental rates
Custom ADU Cons
- Longer construction timeline (4-6 months on-site)
- More variables that can affect final cost
- Requires more decision-making during process
Which is Cheaper? The Real Answer
It depends on your situation.
Prefab may be cheaper if:
- You have easy truck access to your backyard
- Your lot is flat and already prepared
- Standard sizes work for your space
- You want the fastest timeline possible
Custom may be cheaper if:
- You have a sloped or irregular lot
- Delivery access is limited
- You want to maximize every square foot
- Local permitting favors site-built construction
Rental Income Comparison
Custom ADUs often command 10-15% higher rents due to:
- Better integration with the property
- Higher quality finishes
- More appealing curb appeal
- Optimized layouts
Our Recommendation
At One Stop Builders, we specialize in custom-built ADUs because we believe they offer the best value for San Diego homeowners. Contact us at (858) 567-0007 for a free consultation to discuss your options.