LA ADU: Prefab vs Custom vs Garage Conversion — Real 2026 Costs Compared
An accessory dwelling unit in Los Angeles typically lands in one of three cost bands: a garage conversion runs about $80k-$180k, a prefab or modular ADU runs about $180k-$380k all-in, and a ground-up custom detached ADU runs about $280k-$550k+ depending on size, finish level, and lot conditions. Timelines range from roughly 4-8 months for a conversion to 10-18 months for custom ground-up. The right choice depends less on price alone and more on your lot, your goals (rent income vs family vs resale), and how much design control you want.
This guide walks through the three paths honestly, side by side, so you can figure out which one fits your property before you sign anything. All ranges assume LADBS jurisdiction unless noted; separate-city permits in Beverly Hills or Santa Monica add time and cost, and hillside parcels in Bel Air or Pacific Palisades carry their own premiums.
The three paths, in plain English
Under state ADU law, every single-family lot in the state can add at least one ADU. In LA, that shows up as three real-world options:
- Garage conversion: take an existing attached or detached garage and turn it into a legal dwelling. You reuse the slab, the walls, and often the roof.
- Prefab / modular ADU: the unit is built in a factory, trucked to your lot in one or more modules, and set on a foundation your GC pours on site.
- Custom detached ADU: a stick-built, architect-drawn structure built entirely on your lot, like a small custom home.
All three follow the same state ADU law and the same LADBS plan-check process. What changes is how much of the work happens at your address, how much design flexibility you get, and how much the final building is worth on resale.
Garage conversion: cheapest path, biggest constraints
Converting an existing garage is usually the fastest and least expensive route to a legal ADU, and it is the one we recommend most often for owners whose main goal is rental income and who already have a garage in decent shape. Typical all-in cost runs $80k-$180k, and permitted timelines from signed contract to certificate of occupancy typically fall in the 4-8 month range for a straightforward conversion in Mar Vista, West Los Angeles, or Westwood.
The catches: your footprint is fixed by whatever the garage already is (usually 300-500 sq ft), the ceiling height and window placement are constrained, and if the slab or framing turns out to be substandard, the "cheap" path can grow teeth fast. Older detached garages in Brentwood or Bel Air often need slab replacement, new footings, or a full roof rebuild once the inspector opens things up.
For a deeper walkthrough of what typically gets rolled into a garage-conversion budget in LA, see our breakdown at garage conversion ADU cost in Los Angeles.
Prefab / modular ADU: predictable, but not always cheaper
Prefab has been marketed hard in LA over the last few years as the "easy button" for ADUs. The pitch: factory quality control, faster on-site time, fixed pricing. The reality is more nuanced.
Factory time typically runs 8-16 weeks. But before the module can be dropped, you still need site work: soil report, foundation design, foundation pour, sewer/water/electrical trenching, and a crane day. After the module lands, you still need utility hookups, exterior finishes at the seams, interior punch list, and inspections. Total wall-clock time from contract to occupancy usually lands in the 7-12 month range, not the "6-8 weeks" you sometimes see in ads.
Total delivered cost, including site work and utility connections, generally runs $180k-$380k for a 400-800 sq ft prefab. On flat lots in West Los Angeles or Mar Vista, prefab can be genuinely competitive. On a Bel Air hillside or a Pacific Palisades slope, the crane access, retaining walls, and foundation engineering usually push prefab out of its cost advantage — often making custom the smarter buy.
Custom detached ADU: most expensive, most control, best resale
Custom ground-up is what it sounds like: an architect draws a building specifically for your lot, and a general contractor builds it on site. Typical all-in cost runs $280k-$550k+ for a 600-1,200 sq ft unit, and timelines run 10-18 months from first design meeting to final inspection.
You are paying for three things: (1) design that actually fits your lot, your trees, your view, and your main house, (2) finish level and material choices you pick yourself, and (3) resale value — appraisers consistently value custom detached ADUs higher per square foot than converted garages of the same size. On lots in Beverly Hills, Bel Air, or Pacific Palisades, where the main home is already a high-value asset, custom is usually the only path that doesn't drag on the appraisal.
For per-square-foot benchmarks across the range, see ADU cost per square foot in Los Angeles.
Where your neighborhood changes the math
The three cost bands above assume a baseline LADBS lot. Real LA prices swing with jurisdiction and terrain:
- Bel Air (90077): LADBS Hillside Ordinance applies. Grading, retaining, and private-road access typically add 15-25% across all three paths.
- Beverly Hills (90210, 90211, 90212): permits go through the City of Beverly Hills Building & Safety Division, not LADBS. Plan check takes longer and typical premiums land around 20-30%.
- Brentwood (90049): baseline LADBS, but HOA enclaves can add architectural review time. Expect 10-20% over flat-lot baseline.
- Mar Vista (90066): baseline pricing, generally the flattest math in this table (+0-5%).
- Pacific Palisades (90272): LADBS, plus California Coastal Commission review for parcels west of PCH. Premiums typically 15-25%.
- Santa Monica (90401-90405): separate-city permit through Santa Monica City Planning & Community Development, Coastal Commission west of 4th St, rent-stabilization rules for pre-1979 buildings. Premiums typically 10-20%.
- West Los Angeles (90025, 90064): LADBS baseline, with mixed R-1/R-2 zoning that occasionally opens duplex-plus-ADU options — see West LA duplex + ADU zoning under R2.
- Westwood (90024, 90025): LADBS with UCLA-area overlay zones. Premiums typically 5-15%.
Comparing the three paths side by side
| Metric | Prefab ADU | Custom detached ADU | Garage conversion |
|---|---|---|---|
| Typical all-in cost (LADBS baseline) | $180k-$380k | $280k-$550k+ | $80k-$180k |
| Typical size range | 400-800 sq ft | 600-1,200 sq ft | 300-500 sq ft |
| Contract-to-occupancy timeline | 7-12 months | 10-18 months | 4-8 months |
| Design flexibility | Low to moderate (catalog plans) | High (drawn for your lot) | Very low (bound by existing shell) |
| Best on hillside lots? | Usually no (crane + foundation cost) | Yes | Only if existing garage is on level pad |
| Best on flat baseline lots? | Competitive | Yes, if budget allows | Yes, if garage is sound |
| Typical resale premium | Moderate | Highest | Lowest per sq ft, but strong ROI on rent |
| Rental income potential | Strong | Strongest | Solid, capped by size |
How to pick without guessing
A few questions we walk owners through before recommending a path:
- What is your primary goal? Rental income favors garage conversion or small prefab. Multigenerational living favors custom. Resale-driven ADUs almost always favor custom.
- Is your lot flat or sloped? Slopes over roughly 10-15% start to erase prefab's cost advantage and often make custom the better buy.
- What jurisdiction are you in? Beverly Hills and Santa Monica permitting timelines are meaningfully longer than LADBS; that alone can shift which path pencils.
- What is your existing garage actually worth as a shell? If the slab is cracked, framing is undersized, or the roof needs replacement, the "conversion" quickly becomes a rebuild — and a fresh custom build is often cleaner.
For a broader cost overview across ADU types, see our LA ADU cost guide.
Bottom line
There is no "best" ADU path — there is a best path for your lot, your goal, and your neighborhood. Garage conversion wins on speed and cost when you have a sound existing shell. Prefab wins on predictability on flat baseline lots. Custom wins on design, resale, and hillside performance. Anyone quoting you a single number without seeing your lot and asking about your goals is selling, not advising.
Ready to scope your ADU?
Onn has been building on the LA Westside for 20+ years. Design Onn Point holds CSLB license #1133368. We will walk your lot, look at your garage or backyard honestly, and tell you which of the three paths actually fits — including when the answer is "don't build one yet." No pressure, no catalog upsell.