There are five common ways to build an ADU in San Diego: converting an existing garage, building a detached single-story unit, building a detached 2-story unit, installing a modular unit built to state code, or installing a manufactured unit built to federal HUD code. Each one changes your cost, timeline, design flexibility, and what your lot needs to support it. The right choice depends on your lot size, your budget, and whether you want the ADU to look and feel like part of your existing home or you’re optimizing purely for speed.
Below is what each type actually involves, what it costs, and where it runs into trouble — followed by how to think about the decision for your specific property.
Garage conversion ADUs
A garage conversion turns an existing attached or detached garage into livable space instead of building a new structure from the ground up. It’s usually the fastest path to an ADU because the foundation, framing, and roof already exist — the work is mostly interior: insulation, drywall, plumbing and electrical rough-in, windows, and finishes.
Market estimates for a garage conversion in San Diego typically run $120,000 to $180,000, depending on the garage’s condition, whether the slab needs to be replaced, and how far utilities have to travel to reach it. That’s a market range from industry cost data, not an Eco Home Builders quote.
Garage conversions run into two recurring problems. First, older garage slabs are often not thick enough or level enough to meet current residential code, which means a full slab replacement — erasing much of the cost advantage. This comes up often in older San Diego neighborhoods like North Park, South Park, and Kensington, where detached garages tend to predate current foundation standards. Second, converting your garage removes that parking and storage, which some homeowners don’t plan for until it’s gone. If the garage is structurally sound and you don’t need the parking, it’s a strong option. If not, a detached ADU may end up cheaper once you account for the slab work. Either way, the same permit process applies — see our San Diego ADU permit process guide for what that review actually checks.
Detached 2-story ADUs
If your lot doesn’t have enough flat, usable space for a single-story footprint large enough to be worth building, going up instead of out solves the problem — but San Diego caps how far up you can go.
Under San Diego Municipal Code Chapter 14, Article 1, Division 3, detached ADUs and ADUs attached to an existing accessory structure on single-family lots are limited to two stories, and the overall height still has to comply with the maximum structure height allowed in your property’s base zone. In practice, that means a 2-story ADU is legal on most single-family lots in the city, but the exact ceiling depends on your zoning — not a flat citywide number. Confirm your base zone height limit before you design around a specific footprint.
A 2-story ADU also costs more per square foot than a single-story unit of the same size, since it needs a second structural floor, a stairwell, and more complex framing. It’s the right call when lot coverage or setbacks are the binding constraint, not when you simply want more square footage for its own sake. Setback requirements shift once a structure passes 16 feet in height, so review our ADU size, setback, and parking rules guide alongside your zone’s height limit before finalizing a footprint.
Modular ADUs
A modular ADU is built in sections in a factory, then transported and assembled on your property. The distinction that matters: modular units are built to California’s state building code — the same standards used for site-built construction — which means cities generally treat them the same way they treat a stick-built ADU for permitting and inspection purposes.
That code compliance is the main advantage over a manufactured unit. It also means more design flexibility than a manufactured home — modular sections can be configured into a wider range of floor plans and rooflines, so the finished unit doesn’t have to look like a box dropped on your lot.
The tradeoff is lead time on the front end. Because sections are built to order, you’re often waiting on a factory production slot before assembly can even start, and your site still needs a compliant foundation ready before the crane shows up.
Prefab and panelized ADUs
“Prefab” gets used loosely, but it generally covers two approaches: a complete unit built in a factory and delivered whole, or a panelized version where wall and roof panels are manufactured off-site and assembled on your lot. Both promise a shorter on-site construction window than a fully stick-built ADU.
The number that gets marketed is the factory unit price — often $90,000 to $180,000 for the shell alone, depending on size. That’s not the number that matters. Once you add site preparation, a compliant foundation, utility trenching and connections, crane delivery, and permitting, a fully delivered prefab ADU in San Diego typically lands between $150,000 and $350,000 — those figures are sourced from published ADU industry cost data across multiple Southern California builders, not an Eco Home Builders estimate. Prefab looks cheapest at the factory-unit stage and least cheap once your actual lot is factored in.
Prefab makes the most sense on a large, flat, easily accessible lot where a crane and delivery truck can reach the build site without major site work. On a tighter, sloped, or hard-to-access San Diego lot — which describes a lot of the city’s older neighborhoods — the site costs can erase the speed advantage entirely.
Manufactured ADUs
A manufactured ADU is built entirely off-site to federal HUD standards rather than California’s state building code. In California, a manufactured unit used as an ADU has to be placed on a permanent foundation and titled as real property, not personal property, to legally qualify.
Because it’s federally regulated, a city’s control over a manufactured unit’s placement and design is more limited than it is for a modular or stick-built structure — which can simplify some approval steps but also means less flexibility in customizing the floor plan, roofline, and exterior finish to match your existing home. Manufactured units are also typically rectangular, which can make them a visual mismatch on a lot where the ADU sits close to a custom or architecturally distinct primary residence.
Comparing the five types
| Type | Typical cost range* | Timeline advantage | Design flexibility | Best fit |
|---|---|---|---|---|
| Garage conversion | $120,000–$180,000 | Fast — structure exists | Limited by existing footprint | Sound existing garage, parking not needed |
| Detached single-story | Per Eco’s published $375–$600/sf construction rate | Standard | Full custom design | Most flat, mid-size lots |
| Detached 2-story | Higher than single-story per sf | Standard | Full custom design | Small or constrained lots needing more square footage |
| Modular | $150,000–$350,000+ delivered | Faster than stick-built | Moderate — factory sections | Homeowners wanting code parity with site-built |
| Prefab/panelized | $150,000–$350,000+ delivered | Fastest on-site build | Lower — factory shell | Large, flat, easy-access lots |
| Manufactured | Similar tier to prefab | Fast | Lowest — HUD-code shapes | Budget-focused projects where design match matters less |
*Third-party market ranges from published ADU industry cost data, not Eco Home Builders quotes. Eco’s own published construction rate for site-built ADUs is available on our ADU cost guide.
How we approach the decision
Eco Home Builders designs and builds ADUs as site-built, custom structures — garage conversions and detached single- or 2-story units designed to match the architecture, materials, and roofline of your existing home, not a factory shell. That’s a deliberate tradeoff: it takes longer to design than ordering a prefab unit off a spec sheet, but the ADU ends up looking and functioning like it was always part of the property, not an add-on.
If your priority is the fastest possible install on a large, easy-access lot and matching your home’s design is a lower priority, a modular or prefab provider may be the better fit. If you want the ADU to read as part of the main house — inside and out — a site-built design-build approach gives you that control. Our ADU builders San Diego page covers how we run that process end to end. We’ll tell you directly which one makes more sense for your specific lot before you commit to either path.
Frequently asked questions
Planning an ADU and not sure which type fits your lot? Schedule a consultation and we’ll walk your property with you before you commit to a construction method.
