The Great ADU Debate: Convert or Build New?
This is the most common question we hear from homeowners considering an ADU. Both options have distinct advantages, and the right choice depends on your property, goals, and timeline — not just a price comparison.
Garage Conversion ADU
How It Works
You transform your existing attached or detached garage into a code-compliant, permitted, habitable ADU. The existing shell — walls, roof, and foundation — remains, reducing scope compared to new construction.
Key Advantages
Lower total cost
Garage conversions are significantly less expensive than new construction because the structural shell already exists. You're paying for MEP systems, interior finishes, permits, and modifications to the garage door opening — not for framing, roofing, foundation from scratch, or site excavation.
Faster timeline
- Design and permits: 4–8 weeks
- Construction: 8–12 weeks
- Final inspections: 1–2 weeks
Total: typically 4–6 months from contract to move-in, compared to 8–14 months for new construction.
Simpler permitting
Converting an existing structure generally involves less plan-check scrutiny than new construction. Many cities have streamlined garage conversion permitting specifically to encourage ADU creation.
No parking replacement required
California state law (SB 13) does not require replacement parking spaces when a garage is converted to an ADU. You lose your garage, but you don't have to build a replacement parking structure.
Smaller environmental footprint
Reusing an existing structure reduces material consumption and embodied carbon compared to building entirely new.
Key Limitations
Size is constrained by your garage
Your ADU can only be as large as your existing garage footprint — typically 400–600 sq ft for a two-car garage. If your garage is a single-car, you may be working with 200–300 sq ft.
Ceiling height constraints
Standard garages have 8-foot ceilings. Raising the ceiling requires roof structure work that adds cost.
Layout constraints
You're working within existing walls. While walls can be moved, the overall footprint is fixed.
Foundation concerns
Garage slabs are built to support car weight, not habitable space. Depending on thickness and condition, reinforcement may be required.
Loss of garage storage
You'll need alternative storage solutions for items currently in the garage.
Best For
- Budget-conscious homeowners seeking income quickly
- Properties with two-car garages that aren't being fully used
- First-time ADU builders learning the process
- Investors seeking the fastest path to rental income
New Construction ADU (Detached)
How It Works
You build a brand-new structure in your backyard, entirely independent from the main house. Foundation, framing, roofing, and all MEP systems are built from zero.
Key Advantages
Maximum size
California law allows detached ADUs up to 1,200 sq ft — room for two bedrooms, two bathrooms, a full kitchen, and dedicated living space.
Complete design freedom
You choose the floor plan, ceiling height, window placement, exterior architecture, and every finish. No constraints from an existing structure.
Higher rental income potential
Larger, newer units command premium rents. A 900–1,200 sq ft two-bedroom ADU generates higher monthly income than a 500 sq ft garage conversion in the same neighborhood.
Greater property value increase
A larger, purpose-built ADU adds more appraised value than a converted garage, both in absolute dollars and as a percentage of the main home's value.
Keep your garage
You don't sacrifice your parking or storage.
Built to current standards throughout
New construction meets the latest seismic, energy, and building codes in every component — no upgrading of legacy systems.
Key Limitations
Higher total cost
New construction costs significantly more than garage conversion. Foundation, framing, roofing, utility connections, and full MEP systems from scratch add up.
Longer timeline
- Design and engineering: 6–10 weeks
- Permits: 6–12 weeks
- Site work and foundation: 3–4 weeks
- Framing through finish: 12–20 weeks
- Final inspections: 2–3 weeks
Total: typically 8–14 months from design to move-in.
More complex permitting
New construction requires more extensive plan review, engineering calculations, and inspections.
More site disruption
Foundation excavation, concrete trucks, crane lifts for roof trusses, and extended construction duration are more disruptive than a garage conversion.
Yard space required
You'll need adequate rear or side yard to place the new structure while meeting California's 4-foot minimum setback requirements.
Best For
- Homeowners who want maximum rental income over time
- Those planning to house family members long-term in a comfortable unit
- Properties with large backyards and clear setback compliance
- Homeowners who need to keep their garage
- Those planning to eventually sell the ADU separately (under AB 1033)
Side-by-Side Comparison
| Factor | Garage Conversion | New Construction ADU |
|---|---|---|
| Typical timeline | 4–6 months | 8–14 months |
| Typical size range | 400–600 sq ft | 400–1,200 sq ft |
| Design flexibility | Limited by existing footprint | Fully custom |
| Permitting complexity | Lower (existing structure) | Higher (new build) |
| Rental income potential | Strong for size | Highest per unit |
| Site disruption | Lower | Higher |
| Keeps your garage? | No | Yes |
| California parking replacement required? | No (SB 13) | No |
Our Recommendation
If you're on a tighter budget or want to start generating income quickly, a garage conversion is the smarter first move. The faster timeline and lower upfront cost make it excellent for investors focused on return speed.
If you're making a long-term investment and have the budget and lot space, new construction offers superior income, resale value, and design quality.
Some homeowners do both — convert the garage for immediate income, then build a new detached ADU later. California law allows an ADU + JADU on single-family lots, and some cities allow even more flexibility.
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Not sure which is right for your property? Book a free consultation and we'll assess your lot for both options. Or use our cost estimator to compare scenarios for your address before we meet.
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