Conversion vs New Build
Garage Conversion to ADU in California: Costs, Permits, and Whether It's Worth It
Converting a garage into an ADU in California typically costs $80,000 to $160,000 and takes 5 to 9 months including permit time. That sounds cheap compared to a $287K new ADU, but the math is rarely as good as it looks on paper. Most garage conversions end up at the higher end of the range once you account for code-required upgrades, foundation reinforcement, parking replacement (when applicable), and finish quality.
The honest decision framework: convert your garage if the existing structure is structurally sound, in good repair, and you don’t need garage parking. Build a new factory-built ADU instead if the garage has any structural questions, you’d lose needed parking, or you want a home that performs and feels like new construction.
The short version: Garage conversion = $80K to $160K typical, 5 to 9 months. New factory-built ADU = $180K to $557K with a permitting budget included, 4 to 9 months, dramatically better insulation and structure. Conversion wins on price; new build wins on quality, predictability, and resale value.
What “garage conversion” actually involves
A code-compliant garage conversion in California requires every one of these:
- Foundation upgrade. Existing garage slabs are typically thin (3.5”) and unreinforced. For habitable conversion, you’ll often need to demo or reinforce the slab, especially in seismic zones.
- Wall and roof insulation. Garages have R-0 walls. You’ll insulate to current Title 24 standards (R-21 walls, R-30 to R-38 roof). Often requires furring out walls and adding new wall framing.
- Window and door replacement. Garage doors get replaced with a real entry door + windows for code-required egress and natural light.
- Plumbing. A new kitchen and bath means new water supply, drain lines, and a separate sewer connection (or branch from the main house).
- Electrical. A new electrical panel or expanded service. Existing garage circuits are usually not enough for a kitchen + HVAC + bath.
- HVAC. New heating and (in many regions) cooling. Usually a mini-split heat pump.
- Drywall, paint, flooring, cabinetry, appliances, fixtures. Full interior buildout.
- Code compliance for fire, sound, energy. Updated to current 2026 California Building Code + Title 24.
Each of these has a real cost. Add them up honestly and a “$60K garage conversion” turns into $120K once you’re done, sometimes more.
Where the costs actually land
Realistic cost ranges by phase for a typical 400 sq ft California garage:
| Phase | Typical cost |
|---|---|
| Permits, plans, Title 24 | $4,000 to $10,000 |
| Foundation reinforcement / slab work | $8,000 to $25,000 |
| Framing modifications (walls, doors, windows) | $10,000 to $20,000 |
| Insulation + drywall | $8,000 to $15,000 |
| Plumbing (kitchen + bath) | $12,000 to $25,000 |
| Electrical + HVAC | $10,000 to $18,000 |
| Kitchen + cabinets + countertops + appliances | $15,000 to $30,000 |
| Bath fixtures + finishes | $8,000 to $15,000 |
| Flooring + paint + finish work | $5,000 to $10,000 |
| Total range | $80,000 to $168,000 |
The high end of the range applies to: detached garages requiring new utility runs, garages with structural issues (rotted sill plates, foundation cracks), high-cost-of-labor markets (San Francisco, San Jose proper), and finishes that match a new-build quality.
Permits for a garage conversion
California state ADU law treats garage conversions as ADUs, meaning they get the same protections (60-day ministerial review, no owner-occupancy required, no additional parking required within ½ mile of transit).
What that means in practice:
- Cities can’t reject the conversion for being an ADU.
- They CAN require it to meet current building code (insulation, egress, fire protection).
- The 60-day clock starts when you submit a complete application.
- You don’t need to add replacement parking for the converted garage in most California cities (transit exemption applies to almost every urban and suburban lot).
The “parking replacement” question is the one homeowners most often get wrong. Pre-2020, California cities could require you to replace any parking you lost in a garage conversion. State law eliminated that for ADUs within ½ mile of public transit, which covers most of the suburban Bay Area and Monterey region. Your city’s planning department can confirm whether you’re in the transit zone.
Realistic timeline
A garage conversion typically runs:
- Permits: 60 to 90 days (the state’s 60-day rule applies; complex permits take longer)
- Foundation + framing modifications: 4 to 8 weeks
- Mechanicals (plumbing/electrical/HVAC): 3 to 5 weeks
- Insulation + drywall: 2 to 3 weeks
- Finishes (cabinets, appliances, paint, flooring): 4 to 6 weeks
- Inspections + punchlist: 2 to 4 weeks
Total: 5 to 9 months from permit submission to certificate of occupancy. About the same as a new factory-built ADU, but with all the work happening at your property, not off-site.
The honest comparison: convert vs build new
| Factor | Garage conversion | New factory-built ADU (Framework First Six Sixty 1BR) |
|---|---|---|
| Cost | $80K to $160K | $287K, permitting budget included* |
| Build time | 5 to 9 months | 5 months |
| All work happens on your property | Yes (disruption to daily life) | No (97% off-site, 1-day crane delivery) |
| Foundation quality | Reinforced existing slab | New permanent perimeter foundation, engineered for the home |
| Wall framing | 2x4 wall (after furring) | 2x6 exterior walls |
| Insulation R-value | R-21 walls (after upgrade) | R-21 walls (built-in) |
| Square footage flexibility | Limited to garage footprint (~200 to 500 sq ft) | 405 to 1,200 sq ft, your choice |
| Layout flexibility | Garage shape constrains floor plan | Designed-for-purpose floor plan |
| Property value added | ~80% of conversion cost | ~115% of build cost |
| Resale appeal | ”Converted garage” framing | ”Detached ADU” framing |
| Fixed pricing* | No, contractor T&M with allowances | Yes, locked at contract |
*Includes a permitting budget, not final permit costs. Permits and property-specific site work (septic, retaining walls, etc.) vary by property and are confirmed in your feasibility study.
When garage conversion is genuinely the right call
We don’t build garage conversions (we build new factory-built ADUs). But here’s our honest take on when a conversion makes more sense:
- Budget below $150K hard-cap. Even our cheapest model is $180K. If $150K is your absolute ceiling, conversion is the only path forward.
- The existing garage is structurally excellent. Recent construction (2000+), no settling, no water issues, no rotted framing.
- You don’t need the garage for parking. And you’re within ½ mile of transit (no parking replacement required), or you have other parking on the lot.
- You’re okay with a smaller home. Garages cap out around 400 to 500 sq ft. If you want anything bigger, you’d be building an addition AND converting, which loses the price advantage fast.
- You’re handy and willing to general-contractor the project yourself. Owner-managed conversions cost less than hiring a full GC.
When a new factory-built ADU is the better call
- You want a real home, not a converted space. Garage conversions reliably feel like garage conversions, even when well-executed. New ADUs feel like new homes.
- The math doesn’t pencil out. A conversion at $130K with 80% value recovery = ~$104K added to property value. A new $287K ADU with ~115% recovery = ~$330K added. Net property value: conversion -$26K, new ADU +$43K.
- You want to rent it out. Two-bedroom new ADUs rent for $2,400 to $2,800/mo. Converted-garage studios rent for $1,400 to $1,900. The new ADU more than covers its higher mortgage.
- You want predictable cost. New factory-built ADUs price with a permitting budget built in and lock at contract. Garage conversions are time-and-materials with allowances, they run over.
- You need parking. Lots that lose parking via conversion and aren’t in a transit zone can require costly replacement.
What we’d actually recommend
For the average California homeowner asking us “should I convert the garage or build a new ADU?”, we recommend building a new ADU 80% of the time. The financial math works out better, the home is genuinely better, and the predictability (locked price, factory build, off-property construction) eliminates the biggest stressors of a conversion.
For the other 20%, tight budgets, structurally-perfect existing garages, owners who don’t need parking and want the cheapest path to a rentable studio, a conversion can be the right call. We’d recommend hiring a licensed CA contractor who specializes in ADU conversions specifically, not a general remodeler.
If you want the honest answer for your specific situation, start with a free property analysis, we’ll pull your property records, look at your garage, and tell you straight whether conversion or new-build is the better fit. Even if the answer is “convert your garage and don’t work with us,” that’s the honest call.
Comparing your options? Take the 30-second yard-fit quiz to see which Framework First model fits your lot, or run the ROI calculator to compare the financial scenarios.