Detached vs Attached ADU: Choosing the Right Type
Detached ADUs cost $250K-$500K+ and offer maximum privacy and rental potential. Attached ADUs cost $150K-$350K and are easier to connect to existing utilities. Your lot size, budget, and privacy needs determine the best choice.
Should I build a detached or attached ADU?
Build detached if you have lot space and want maximum privacy, rental income, and design freedom. Build attached if your lot is tight, your budget is lower, or you want easier access between the main house and the ADU. Detached ADUs cost $250K-$500K+; attached ADUs cost $150K-$350K.
Two ADU Layouts, Different Trade-offs
Once you have decided to build an ADU, the next question is whether to build it as a separate structure on your property or attach it to your existing home. Both are permitted under California state law, and both create independent living units. The differences come down to cost, privacy, lot requirements, and long-term value.
Quick Comparison
| Factor | Detached ADU | Attached ADU |
|---|---|---|
| Cost | $250,000-$500,000+ | $150,000-$350,000 |
| Timeline | 8-14 months | 6-10 months |
| Privacy | Full separation | Shared wall(s) |
| Rental Income | $2,500-$4,000/mo | $2,000-$3,000/mo |
| Utility Connections | New, independent lines | Can share with main house |
| Lot Requirement | Needs adequate yard space | Uses existing home footprint |
| Best for | Rental income, in-law suites | Smaller lots, family use |
Detached ADUs: Maximum Independence
A detached ADU is a standalone structure built on your property, separate from the main house. It has its own entrance, its own outdoor space, and complete physical separation from the primary residence.
Advantages:
- Complete privacy for both occupants and tenants
- Higher rental income due to the independent feel
- Full design flexibility (layout, orientation, windows on all sides)
- No construction impact on the main house during building
- Greater property value increase at resale
Considerations:
- Higher cost due to new foundation, framing, roofing, and utility connections
- Requires sufficient lot space with setback compliance
- Longer permitting and construction timeline
- New utility connections add $15,000-$30,000
Design Options for Detached ADUs
Detached ADUs can range from compact 400 sqft studios to spacious 1,200 sqft two-bedroom units. Common configurations include a one-bedroom with open living area (600-800 sqft), a two-bedroom with kitchen and living room (900-1,200 sqft), and studio layouts for rental efficiency (400-500 sqft).
Attached ADUs: Cost-Effective and Connected
An attached ADU shares at least one wall with your main house. It can be built as a side addition, a rear addition, or a conversion of existing attached space (like converting a portion of a large home into an independent unit).
Advantages:
- Lower construction cost (shared wall reduces materials and labor)
- Shared utility connections save $15,000-$30,000
- Works on smaller lots where a detached structure would not fit
- Shorter construction timeline
- Easier access between the main home and the ADU (useful for aging parents)
Considerations:
- Shared walls mean less sound privacy
- Layout is constrained by the existing home structure
- Construction may temporarily affect the main house
- Lower rental rates compared to detached units
- Tenants may feel less independent
Design Options for Attached ADUs
Attached ADUs typically range from 400 to 800 sqft. Common configurations include a studio with kitchenette (400-500 sqft), a one-bedroom unit accessed from the side of the home (500-700 sqft), and a rear addition with its own entrance and small patio (600-800 sqft).
Which Type Earns More Rental Income?
Detached ADUs consistently command higher rents in the Bay Area. The privacy factor is significant: tenants pay a premium for a unit that feels like its own space rather than an attached room.
Typical Bay Area rental rates (2026):
- Detached ADU (1-bed, 600-800 sqft): $2,500-$4,000/month
- Attached ADU (1-bed, 500-700 sqft): $2,000-$3,000/month
Over 10 years, the rent difference ($500-$1,000/month) amounts to $60,000-$120,000 in additional income. This often justifies the higher upfront cost of a detached unit.
Making the Right Choice for Your Property
Choose a detached ADU if:
- You have a lot with adequate space behind or beside the main house
- Rental income is a primary goal
- Privacy for the occupant (and yourself) matters
- Your budget accommodates $250K-$500K+
- You want the highest long-term property value increase
Choose an attached ADU if:
- Your lot is too small for a detached structure
- Your budget is under $250K
- The ADU is for a family member who benefits from proximity
- You want a faster, less disruptive project
- You prefer to share utility connections and reduce infrastructure costs
How Custom Home Designs Both Types
Custom Home designs detached and attached ADUs through the same two-phase process. In Phase 1, we evaluate your lot, assess the existing home, and design the ADU that best fits your property and goals. For attached ADUs, this includes a structural analysis of the shared wall. For detached ADUs, it covers site planning, utility routing, and placement optimization.
Our 3D visualizations show exactly how the ADU relates to your main home and outdoor space, so you see the full picture before construction begins.
Take the First Step
The best way to determine which ADU type works for your property is a site assessment. We will measure your lot, review setback requirements, and recommend the approach that delivers the most value.
Schedule a free consultation to discuss your ADU project with Custom Home.
Frequently Asked Questions
Which type has better resale value?
Detached ADUs typically add more resale value because they function as fully independent units. Appraisers treat them more like separate structures with their own income potential. Attached ADUs add value but are viewed more like additional rooms or in-law suites.
Do attached ADUs need separate utility connections?
Attached ADUs can often share utility connections with the main house, which reduces cost. However, California law requires separate electrical panels for metering purposes. Detached ADUs require independent utility connections for water, sewer, and electrical, adding $15,000-$30,000 to the project.
What are the setback requirements for each type?
Both types follow the same California state setback rules: 4 feet from side and rear property lines. Attached ADUs must also comply with the main house setbacks since they are part of the same structure. Detached ADUs have more placement flexibility on the lot.
Which is better for renting out?
Detached ADUs are better for rental purposes. The physical separation provides privacy for both the tenant and the homeowner, which supports higher rents ($2,500-$4,000/mo vs $2,000-$3,000/mo for attached). Tenants strongly prefer detached units when given the choice.