One of the first big decisions for a San Diego ADU is prefab (modular) vs. custom (site-built). Both are great options, but they differ on cost predictability, timeline, and how much you can customize. Here is how to choose.
Quick comparison
| Factor | Prefab / Modular ADU | Custom / Site-Built ADU |
|---|---|---|
| Cost predictability | High (fixed model pricing) | Varies more with design and site |
| Build speed | Faster (built in a factory while site is prepped) | Slower (built entirely on site) |
| Customization | Limited to available models and options | Fully customizable to your lot and taste |
| Best for tricky lots | Harder (needs crane and clear access) | Easier (built in place) |
| Typical San Diego cost | Often more predictable per the model | Roughly $350 to $550 per sq ft |
For full pricing by ADU type, see our San Diego ADU Cost Guide.
What is a prefab (modular) ADU?
A prefab or modular ADU is built in sections at a factory, then transported and craned onto a foundation at your property. Because most of the construction happens off site, prefab ADUs can be installed quickly once the foundation and utilities are ready, and pricing is usually fixed by model. The tradeoff: you choose from set floor plans and options, and your lot needs clear access for delivery and a crane.
What is a custom (site-built) ADU?
A custom ADU is designed and built entirely on your property, like a small home. You control the layout, finishes, and how it fits your lot, which is ideal for unusual shapes, slopes, or matching your main house. The tradeoff: it typically takes longer and the final cost depends more on design choices and site conditions.
When prefab makes sense
- You want a faster, more predictable timeline and budget.
- Your lot has clear access for delivery and a crane.
- A standard floor plan fits your needs.
When custom makes sense
- Your lot is tight, sloped, or oddly shaped.
- You want a specific design or to match your existing home.
- Maximizing every square foot matters more than speed.
San Diego context
Both prefab and custom ADUs follow the same San Diego permit process and fee rules, including the impact-fee waiver for units 750 sq ft or less. See our San Diego ADU Permit Guide for the full breakdown.
Not sure which is right for you?
Many San Diego builders offer both, and the best way to decide is to compare options for your specific lot. Get matched with up to 3 vetted San Diego ADU builders and ask each how prefab and custom would work for your property.