# Detached vs Attached ADUs: Pros and Cons
Detached ADUs (separate structure in your backyard) offer maximum privacy and typically rent for 10-20% more than attached ADUs. Attached ADUs (added to your existing house) cost 20-30% less to build and are simpler to permit and construct. Neither is better - the right choice depends on your property layout, budget, and who will use the ADU.
Most homeowners choose detached when they have the space and budget for it. But attached ADUs make perfect sense when you have limited yard space, tight budget, or your family member will use it (where privacy matters less than convenience).
## Detached ADUs: The Backyard Cottage
**What it is:** Completely separate structure in your backyard, typically 20-30 feet from your main house.
**Who chooses this:**
- Homeowners with decent backyard space (2,000+ sq ft yards)
- Anyone planning to rent long-term
- Families who want clear separation from main house
- Properties where attached addition would look awkward
**The privacy advantage:**
Detached ADUs feel like living in your own space, not someone else's house. Tenants prefer this - they can come and go freely, make noise at normal hours, and feel genuinely independent. Your family members get privacy too, which matters for adult children or aging parents who want autonomy.
**Real homeowner experience:**
"We built a detached 600 sq ft ADU. Our tenant loves it because she cannot hear us and we cannot hear her. When she is grilling on her patio or has friends over, it does not affect us at all. Worth the extra cost." - Portland homeowner
**Cost range:** $180,000-$300,000 for typical 500-800 sq ft ADU
**Why it costs more:**
- Separate foundation and structure from scratch
- Independent utility connections (water, sewer, electrical, gas)
- Often requires more site prep (grading, pathways, landscaping)
- Cannot share any existing walls, roof, or systems with main house
## Attached ADUs: The Added Wing
**What it is:** Addition built onto your existing house - might be above garage, side addition, or bump-out from main house.
**Who chooses this:**
- Limited backyard space or narrow lots
- Tighter budgets (typically 20-30% cheaper than detached)
- Family members will use it (parent, adult child)
- Properties where lot coverage limits prevent detached structure
**The cost advantage:**
You are leveraging existing structure. Often share at least one wall with main house, sometimes utilities run through shared spaces, foundation work can be simpler if building off existing footprint.
**Cost range:** $140,000-$220,000 for typical 500-800 sq ft ADU
**Real homeowner experience:**
"We converted space above our garage into a 550 sq ft ADU for my dad. Cost $165,000 vs $220,000+ for detached. He is right there when we need him but has his own entrance. Perfect for family." - Denver homeowner
**The convenience factor:**
For family use, being attached to the main house is actually a feature. Aging parent needs help? You are right there. Adult child wants to join family dinner? Thirty seconds from their door to yours.
## Privacy Comparison
**Detached wins for:**
- Long-term rentals where tenant wants true independence
- Adult children establishing their own household
- Situation where noise could be an issue (musicians, different schedules)
- Visual separation (ADU occupants cannot see into main house windows)
**Attached works fine for:**
- Family members who want connection more than separation
- Short-term rentals (guests expect less privacy)
- Owner-occupied situations (you live in ADU, rent main house)
- Quiet tenants or families with compatible schedules
**The sound factor:**
Detached: You typically cannot hear normal living sounds between buildings. Loud music or parties, maybe, but day-to-day life is silent.
Attached: With proper insulation and sound barriers, you will not hear normal conversation, but you might notice TV, footsteps, or plumbing sounds. Good construction minimizes this, but it is never truly silent like detached.
## Property Layout Considerations
**Your lot might require attached if:**
- Backyard is less than 1,500 sq ft
- Setback requirements leave no room for detached structure
- Side yard has space but backyard does not
- Lot coverage maxes out with main house (attached does not always count toward coverage limits)
- Steep slope makes detached foundation expensive but attached could step down from main house
**Detached makes sense if:**
- You have 2,000+ sq ft backyard
- Yard is flat or gently sloping
- Good solar access in backyard
- Main house does not have good spot for addition
- You value maximizing privacy
**Check your local rules:** Some cities have different regulations for attached vs detached (Portland allows both but has different size limits for each).
## Construction Timeline
**Detached:** 8-12 months typically. Longer because you are building from bare ground up.
**Attached:** 6-10 months typically. Can move faster because you are working off existing structure and utilities are closer.
**Disruption during construction:**
Detached: Construction happens in backyard. Less impact on living in main house during build. You might lose yard access but your home life stays relatively normal.
Attached: Construction happens right on your house. More dust, noise, and impact. Might need to vacate part of your house or deal with construction workers walking through.
## Rental Income Potential
**Detached ADUs typically rent for 10-20% more** because:
- Feels more like "your own place"
- Better privacy attracts quality tenants willing to pay premium
- Separate outdoor space (patio or deck) is possible
- Appeals to wider range of renters
**Example:**
- Detached 600 sq ft ADU: $1,700 per month
- Attached 600 sq ft ADU: $1,500 per month
The gap narrows in tight rental markets and disappears entirely for short-term rentals (Airbnb guests care less about detached vs attached).
## Resale Value Impact
**Detached ADUs often add more to home value** because:
- Future buyers see more options (rental, family, multi-generational, they could live there)
- Feels like separate property
- Does not change character of main house
- Can be marketed as "main house plus cottage"
**Attached ADUs still add significant value** but sometimes get perceived as "addition with separate entrance" rather than true ADU. The value boost is real but sometimes 10-15% less than equivalent detached ADU.
**Both add substantial value** - we are talking $100,000-$200,000 increase either way. The difference between them matters less than the difference between having an ADU vs not having one.
## Utility Considerations
**Detached requires:**
- Trenching to run utilities from main house or street
- Separate electrical panel (typically)
- Water and sewer lines across yard
- Sometimes separate meters for utilities
**Added cost:** $10,000-$20,000 for utility runs depending on distance and conditions.
**Attached benefits from:**
- Shorter utility runs
- Often share existing panels or systems
- Simpler connections
- Sometimes share meters (check local rules on submetering)
**Savings:** $5,000-$15,000 in utility connection costs.
## Above-Garage ADUs: The Hybrid Option
A special type of attached ADU built over your garage. It offers:
**Advantages:**
- Uses space that is currently just roof
- Separate entrance from main house (more privacy than typical attached)
- Usually counts toward detached setback rules, not attached rules
- Costs less than detached because foundation exists
**Limitations:**
- Only works if garage has enough ceiling height or you can raise roofline
- Structural reinforcement often needed (garage was not built for living space above)
- Limited by garage footprint size
- Noise from garage can be issue (insulation helps)
**Cost range:** $150,000-$240,000 for 500-700 sq ft
**Best for:** Properties with detached garage that is 10+ feet from main house. Gets you detached privacy at partial detached cost.
## Permitting Differences
**Some cities have different rules for attached vs detached:**
- Maximum size limits may differ
- Setback requirements often stricter for detached
- Parking requirements sometimes differ
- Owner-occupancy requirements (rare) might apply only to detached
**Most cities treat them the same** for permit timeline and process, but check your local ADU ordinance or ask builders familiar with your area.
## Making Your Decision
**Choose detached if:**
- You have adequate backyard space
- Budget supports it ($180K-$300K range)
- Planning to rent to non-family
- Privacy is high priority
- Your lot layout clearly works for separate structure
**Choose attached if:**
- Limited yard space or narrow lot
- Tighter budget ($140K-$220K range)
- Family member will live there
- Your main house has logical place for addition
- Want simpler, faster construction
**Consider above-garage if:**
- You have detached garage with room above
- Want detached privacy at mid-range price
- Garage structure can support second story
## Do Not Overthink It
Here is the truth: Both types of ADUs make homeowners happy. Detached gets you more privacy and rental income. Attached saves money and construction complexity.
**The deciding factor is usually your property** - if you have 3,000 sq ft backyard, detached is obvious. If you have 1,200 sq ft backyard, attached makes more sense. If you have 2,000 sq ft backyard, either could work.
**Next step:** Check if your property qualifies for an ADU, then talk to 2-3 local builders who can walk your property and tell you what actually makes sense for your specific lot. They have seen hundreds of properties and can spot whether detached or attached is the natural choice.
Most homeowners find the decision is clearer than they expected once they see their property through a builder's eyes.