Types of ADU in California — All 6 Variants Compared (2026)
Six ADU types. Each has a different cost band, timeline, and use case. Pick wrong and the project costs 30% more. California recognizes six distinct ADU types under state law (Gov Code §65852.2 and §65852.22): detached, attached, garage conversion, JADU, movable ADU, and prefab. Each unlocks a different slice of state-law protection. Choosing the wrong type can add $30K–$80K of unnecessary cost or 3–6 months of unnecessary timeline. Sometimes the analysis points to the answer that no type makes sense on this lot — and we say that clearly, before any money moves.
6 types comparedCost · timeline · use caseState-law specificsDecision tree included
ADUscale's homeowners are not "ADU buyers" in the abstract. They land on this page from one of four distinct decision postures, and the right type usually follows the posture.
Profile A · 55–65
The Aging-In-Place Planner
Choosing for a parent or for themselves at retirement. Privacy, accessibility, and predictable construction noise matter more than rental yield. JADU is most-considered (kept inside the main house) but owner-occupancy and the kitchenette ceiling rule it out for many. Detached is second, especially when wheelchair turn-radius or no-step thresholds are required.
Trap to avoid: Picking JADU because it sounds simple, then learning the kitchenette + shared-bath rules don't fit a parent who cooks and showers on the same schedule as you do.
Profile B · 40–55
The Equity Optimizer
Choosing for rental income or property-value lift. Detached new construction or prefab usually wins on rent-to-cost ratio. Garage conversion wins on speed and total cost when the existing garage is structurally sound.
Trap to avoid: Picking the cheapest path (garage conversion) when the garage is too small or too compromised to support the rental rate that justified the project — the math falls apart at the rent comp.
Profile C · 40–55
The Recent Mover
Bought in the last 5–10 years. Choosing under the most pressure of the four profiles, because the original purchase price compressed the equity buffer. Garage conversion or prefab tend to fit the budget; detached often doesn't.
Trap to avoid: Stretching to detached because rental projections justify it on paper, then getting hit with hillside or sewer-lateral costs ($20K–$60K hillside soils, $15K–$30K sewer-lateral) that the projection assumed away.
Profile D · any age
The First-Timer
Has never managed construction. Most exposed to type confusion because the legal differences across the six types are not intuitive. JADUs vs ADUs in particular catch First-Timers off guard (owner-occupancy, kitchenette ceiling).
Trap to avoid: Starting architectural design before confirming which types your specific lot supports under state and local rules — the architect bills typically open at 5–10% of project cost, and that money is hard to recover if the type changes mid-project.
The Reality Check returns the type recommendations specific to your lot in two minutes, calibrated to all four sub-profile risk patterns above.
Section 03
Quick Comparison — All 6 ADU Types
Cost and timeline are the two variables that move first when you change the type. State-law protection is the third — and it's the one homeowners are most likely to misread on JADU and MADU.
TypeCost range (CA, 600 sqft)Timeline · Cost
01. Detached new construction$180K–$330K · 8–13 mo
02. Attached addition$180K–$300K · 7–12 mo
03. Garage conversion$150K–$270K · 4–7 mo
04. Junior ADU (JADU)$80K–$160K · 3–6 mo
05. Movable ADU (MADU)$70K–$180K · 2–4 mo
06. Prefab / manufactured$120K–$240K · 3–6 mo
Best for · State-law protection
01. Detached
Maximum independence + privacy, rental units
Full SB 1069 / AB 68 protection
02. Attached
Smaller lots, older homes, shared infrastructure
Full state protection
03. Garage
Lowest cost, existing garage, fastest path
Full + parking-replacement waiver
04. JADU
Inside main house, kitchenette only, owner-occupied
Different rules; owner-occupancy required
05. Movable
Tenant flexibility, lower cost, factory-built
Limited; local rules vary
06. Prefab
Faster construction, cost predictability
Full if HUD or CA-code certified
Mid-stage commit. If the Reality Check has narrowed your lot to two or three types and you want a parcel-level cost band and risk profile before architectural plans, the $199 Feasibility & Risk Assessment does that work. Roughly 1 in 7 reports recommend not to build at all on the lot as scoped — that is also a useful answer.
Each type below: what it is, why you'd choose it, why you'd skip it, the 2026 California cost band, and the deep-dive routes that follow.
Type 01
Detached new construction
A standalone structure built in the rear yard, separate from the main house. Most common type for rental units and aging-parent suites.
Why choose
Maximum privacy and independence (separate entrance, separate utilities possible)
Best rental income potential
No shared walls — better acoustic and visual separation
Most flexible layout options
Why skip
Highest cost per sqft ($300–$550/sqft, industry cost-benchmark data)
Longest timeline
Requires meaningful rear-yard space (typically 600+ sqft buildable footprint)
More site prep, foundations, utility runs
Cost band (CA, 2026): $180K–$330K for 600 sqft. $250K–$400K with site complications (sewer-lateral $15K–$30K, hillside soils $20K–$60K, HPOZ design overhead).
A small unit (max 500 sqft) carved out of the main house, with its own entrance and kitchenette but sharing some infrastructure. A different category from a regular ADU, with different rules.
Why choose
Lowest cost ($80K–$160K typical)
Fastest path (3–6 months)
Kitchenette (not full kitchen) is sufficient under state law
Works when neither attached addition nor detached construction is feasible
Why skip
Owner-occupancy is still required (one of the two units must be owner-occupied)
500 sqft maximum size
Cannot have a full second kitchen; kitchenette only
Bathroom may be shared with main house in some configurations
Cost band (CA, 2026): $80K–$160K for typical JADU (200–500 sqft, kitchenette + bath). This is the type most often misunderstood by homeowners.
A factory-built unit on a chassis installed on your property. Distinct from prefab modular construction because MADUs are not permanently affixed to a foundation in the same way.
Why choose
Fastest installation (2–4 months)
Lower upfront cost
Can be relocated if your situation changes
Some California cities have streamlined MADU permits
Why skip
Limited state-law protection (rules vary by city)
Some cities don't allow them as ADUs at all
Lower rental income potential than permanent ADUs
Property tax treatment and financing can differ from permanent construction
Cost band (CA, 2026): $70K–$180K including delivery + installation. $50K–$120K for the unit itself, $20K–$60K for site prep + utility connections.
A factory-built unit delivered to your property and installed on a permanent foundation. Distinct from MADU, because prefab ADUs are permanently affixed and treated as standard construction for most purposes.
Why choose
Faster construction timeline (3–6 months total; much of the build happens in the factory)
More predictable cost (factory production reduces field-labor variability)
Quality-control consistency
Some manufacturers handle the entire process (design, permit, manufacture, install)
Why skip
Site preparation costs can erase savings (hillside soils $20K–$60K can absorb factory savings)
Customization is limited (you choose from manufacturer's models)
The four questions that route most California homeowners to the right type. Run them in order; each answer narrows the path.
1
Do you already have a garage?
YESAttached or detached, in good structural condition → Garage conversion is usually the lowest-cost path. Skip to the garage-conversion deep dive.
NOOr yes but in poor structural condition → Continue to Question 2.
2
Do you have rear-yard space for a 600+ sqft footprint?
YESWith adequate setbacks → Detached new construction or prefab. Continue to Question 3 to choose between them.
PARTIALBuildable side or rear bump-out area → Attached addition is typically the answer.
NONo buildable footprint at all → JADU may be the only path. Skip to the JADU deep dive.
3
Do you prioritize speed and cost predictability over customization?
YESFastest path matters → Prefab is usually the answer.
NOYou want a one-of-one design → Detached new construction is the answer.
4
Are you renting to a tenant for the long term?
YESLong-term rental → Detached new construction or prefab provide the best rental economics.
NOFor family or temporary use → Garage conversion, JADU, or MADU may be cheaper paths that don't sacrifice on the use case.
Run the Reality Check for personalized type recommendations. The four-question filter narrows the field; the Reality Check confirms which types your specific lot legally supports under California state law and your city's local rules.
Section 06
Citable Factoids — California ADU Types
The numbers and statutory references that show up across the type-comparison conversation.
JADU is typically cheapest ($80K–$160K), then garage conversion ($150K–$270K), then prefab ($120K–$240K), then attached addition ($180K–$300K), then detached new construction ($180K–$330K). Movable ADU sits in a similar cost band to JADU but with very different state-law treatment.
Movable ADU (MADU) is fastest at 2–4 months. JADU and prefab follow at 3–6 months. Garage conversion at 4–7 months. Attached and detached new construction at 7–13 months.
Yes. California state law (AB 1033 (2023), SB 1211 (2024), and prior reforms) allows multiple ADUs in many configurations. Single-family lots can typically have one ADU + one JADU. Multi-family properties can now have up to 8 detached ADUs or 25% of existing units under SB 1211. Each city implements the state framework slightly differently.
Detached new construction typically generates the highest rental income because it offers the most privacy and independence. Garage conversions and prefab are competitive on rental income per dollar invested. JADUs and MADUs typically rent for less due to size and integration with the main house.
Yes. Basement conversion is a form of attached ADU under California state law. Cost varies dramatically based on whether the basement was already finished, ceiling height, light and egress availability, and waterproofing. Common in Bay Area and older urban housing stock; less common in single-story sprawl areas.
Sometimes, depending on the city and the specific definition used. California state law has a 'movable ADU' framework, but local implementation varies. Some cities treat them as ADUs; others treat them as RVs. Check your city's specific rules.
ADU is a separate dwelling unit on the property: full kitchen, full bath, separate entrance, can be on its own meter. JADU is a unit carved out of the main house: kitchenette (not full kitchen), separate entrance, max 500 sqft, owner-occupancy required. They're treated as different categories under California law.
Yes. California state law allows one ADU + one JADU on most single-family lots. The two units are governed by different rules (JADU requires owner-occupancy; the ADU does not). Combining them is sometimes the right answer for multigenerational housing.
The free Reality Check returns which of the six types your specific lot supports under California state law and your city's local rules. The $199 Feasibility & Risk Assessment narrows the cost band, flags the change-order categories most likely to hit your lot, and reads the contractor market for the type you're considering. Roughly 1 in 7 reports recommends not to build on the lot as scoped.
Pick the right type
The right ADU type depends on your lot, budget, timeline, and use case.
Sometimes the right answer is that no type makes sense on this lot, with this budget, in this market — and we say that, before any money moves and before the first 5–10% of the budget goes to architectural plans. The Reality Check returns the type recommendations your specific lot supports in two minutes.
