Detached vs attached ADU in Boise — which is right?
Detached if your lot fits one. Attached if it doesn't, or if you specifically want to share utilities and main-house infrastructure to keep cost down. For most Boise homeowners, detached delivers a meaningfully better rental, resale, and privacy outcome at a comparable cost — which is why we recommend detached as the default and attached as the considered fallback.
Quick comparison
| Factor | Detached | Attached |
|---|---|---|
| Privacy (both directions) | High | Low to medium |
| Rent ceiling (1BR) | $1,500-$1,800/mo | $1,200-$1,500/mo |
| Build cost | $115k-$345k (pre-approved) | $60k-$200k (varies widely) |
| Per-sqft construction cost | Higher (new foundation, full enclosure) | Lower (reuses existing structure) |
| Resale lift | Highest | Modest |
| Layout flexibility | High (within 900 sq ft cap) | Constrained by existing structure |
| Permitting timeline | Same (3-5 wk pre-approved) | Same (3-5 wk pre-approved) |
When detached wins
- Lot fits a 280-695 sq ft detached footprint with required setbacks
- Privacy matters (rental, long-term tenant, aging-parent suite)
- Resale upside matters (most appraisers treat detached as a separate dwelling)
- Lot has alley access or generous side-yard access for construction
- Homeowner wants exterior elevation freedom (different roof, different siding)
When attached wins
- Lot can't fit a detached footprint (substandard width, easement-eaten)
- Existing basement is full-height, dry, and roughed-in for plumbing
- Budget caps below ~$120k turnkey
- Homeowner specifically wants shared utility infrastructure
- Goal is a guest space or office, not a rental
Privacy is the underrated factor
Attached ADUs share a wall — sometimes a floor — with the main house. Even with sound-rated assemblies, daily life leaks through. For owner-use (guest space, occasional family) attached is fine. For long-term rental, privacy drives tenant retention and rent ceiling more than any other variable. Detached units rent for $200-400 more per month in Boise on equivalent square footage, partly because tenants pay a clear privacy premium.
Resale and appraisal differences
Ada County appraisers generally treat permitted detached ADUs as separate dwelling units, contributing close to standalone value at appraisal. Attached ADUs (basement conversions, additions) typically add square-footage value to the main residence rather than reading as a second dwelling. The practical effect: $50-80k of additional appraised value on a detached vs. an attached of equivalent size, in most established Boise neighborhoods.
Cost reality check
Attached often looks cheaper on the surface but the gap narrows in practice. A basement conversion that needs egress windows, fire separation, separate HVAC, and code-compliant ceiling height can run $80-150k — close enough to a small detached Goldfinch ($115k turnkey) that the privacy and resale math usually favor detached. See our ADU cost guide for detailed ranges.
Where to next
Most homeowners reading this end up shopping detached. Useful next reads: backyard cottage options (the homeowner-friendly framing of detached ADUs), Boise's six pre-approved plans (the fastest detached path), and the design-build contractor overview. When you're ready, the free ADU lot check confirms which configuration actually fits your lot.