Seattle DADU vs. Backyard Cottage in 2026: Which One Actually Pencils for Puget Sound Homeowners?

Homeowners hear four terms for what feels like the same building: ADU, DADU, backyard cottage, and (since 2024) middle-housing unit. Here's how Seattle SDCI, Tacoma, Bellevue, King County, and Pierce County actually use them in 2026.

ADU (Accessory Dwelling Unit) is the legal umbrella — any secondary dwelling on a lot with a primary house. It covers attached ADUs (basement conversions, additions) and detached ADUs.

DADU (Detached Accessory Dwelling Unit) is the code term for a standalone backyard structure. Seattle allows up to 1,000 sq ft interior floor area, 18–25 ft height (zone-dependent), with 5-ft side/rear setbacks.

Backyard cottage is SDCI's public-facing marketing name for a DADU — used interchangeably on aduniverse.seattle.gov. There is no separate permit class.

HB 1110 middle-housing units are a different animal — those are 2–4 primary units on one lot under Washington's 2023 middle-housing law, not accessory to a house. If you want a DADU + main house, you're in ADU territory, not HB 1110.

Numbers below are turnkey averages from NW ADU Builders 2025–2026 projects and cross-referenced with the ADUniverse cost bands and King/Pierce County permit-fee schedules. Includes permits, site work, standard finishes, PSE hook-ups. Excludes land, geotech surprises, and sewer laterals >40 ft.

| City | Small Cottage (400–550 sq ft studio) | Standard DADU (700 sq ft, 1BR) | Full DADU (900–1,000 sq ft, 2BR) |

|---|---|---|---|

| Seattle (Ballard, West Seattle, Beacon Hill) | $155K–$185K | $195K–$235K | $255K–$310K |

| Bellevue (Crossroads, Lake Hills) | $175K–$205K | $215K–$255K | $275K–$330K |

| Tacoma (North End, Stadium) | $135K–$165K | $165K–$200K | $215K–$260K |

| Renton (Highlands, Kennydale) | $145K–$175K | $180K–$215K | $230K–$275K |

| Kent (East Hill, West Hill) | $140K–$170K | $175K–$210K | $220K–$265K |

Takeaway: the small-cottage path is roughly 25–30% cheaper than a full DADU in every city — but rent scales only 15–20% lower. That's why studios post the strongest cash-on-cash in Tacoma, Kent, and Renton.

Small cottages using ADUniverse pre-approved plans (Seattle) or the equivalent King County pre-approved catalog clear permits fastest — 4–6 weeks in Seattle, 5–8 weeks unincorporated King County. See our full ADUniverse guide for which of the 12 plans fit each lot.

Custom DADUs in Seattle currently run 4–7 months for permit review, sometimes longer on ECA lots. Tacoma custom DADUs: 10–14 weeks. Bellevue: 12–16 weeks. Renton and Kent: 8–12 weeks — Renton and Kent are the fastest large-city permit jurisdictions in Puget Sound in 2026.

AI permit pre-screening (covered in our DADU permit AI guide) is now cutting Seattle custom-DADU review by ~40%, closing the gap with the pre-approved cottage path.

From our 40-backyard 2026 rental study, median long-term rent by size and city:

| Unit | Seattle | Bellevue | Tacoma | Renton | Kent |

|---|---|---|---|---|---|

| Small cottage (500 sq ft studio) | $1,850 | $2,150 | $1,450 | $1,650 | $1,600 |

| Standard DADU (700 sq ft, 1BR) | $2,600 | $3,100 | $1,950 | $2,250 | $2,150 |

| Full DADU (950 sq ft, 2BR) | $3,050 | $3,550 | $2,300 | $2,600 | $2,500 |

Cash-on-cash winners: Small cottages in Tacoma North End and Kent East Hill post 9.1–9.6% cash-on-cash in 2026 — highest in the region. Full 2-BR DADUs in Bellevue and Kirkland hit $3,550/mo but drop to 6.9% cash-on-cash because of the build premium and King County assessed-value lift.

STR angle: Only DADUs (not sub-500 sq ft studios in most cases) meaningfully outperform on short-term rental in the three Puget Sound STR-friendly submarkets — Ballard, Capitol Hill, and downtown Bellevue — and only when the ADU sits on the host's primary-residence lot under Seattle's 2026 STR ordinance.

Small cottages ($145K–$210K) typically fit inside a straight HELOC from BECU, Sound Credit Union, or WSECU — no second-lien renovation loan required. Most Puget Sound homeowners with $180K+ tappable equity can fund a cottage without touching their primary mortgage.

Full DADUs ($235K+) usually need the stacked financing playbook — HELOC + Fannie Mae HomeStyle Renovation, or RenoFi's after-completion-value lending. See our full 2026 DADU financing stack for lender-by-lender terms.

The WSHFC House Key ADU program (up to $85K for income-qualified households across King, Pierce, and Snohomish counties) works for either path but is most impactful on cottage-sized builds, where it can cover 40–50% of hard cost.

Both a small cottage and a full DADU qualify for the 2026 Puget Sound heat-pump rebate stack ($2,400–$4,000 from PSE, plus federal IRA credits — full rebate breakdown here). The rebate is per-unit, not per-square-foot, so a $175K cottage gets the same $10K–$13K stack as a $290K full DADU — a proportionally bigger cost reduction on the smaller build.

In 2024–2026 King County comps, a permitted DADU adds $190K–$260K to a Seattle single-family home sale (Zonda + Redfin ADU comp studies). A small backyard cottage under 550 sq ft adds $120K–$165K. Per-square-foot, the cottage often wins — but the absolute lift favors the full DADU by $70K–$100K.

The unit-lot subdivision path (see our WA unit-lot subdivision playbook) is available for full-sized DADUs on qualifying lots — sub-500 sq ft studios usually do not qualify.

In Seattle (King County, WA), the DADU vs backyard cottage decision is really a lot-size and zoning question. On tight Ballard, Fremont, and Wallingford NR2 lots (3,200–4,800 sq ft), a small ADUniverse cottage (480–650 sq ft) fits inside setbacks without a variance and hits SDCI's expedited 4–6 week track. On larger West Seattle (Alki, Admiral), Beacon Hill, Columbia City, Rainier Valley, and Wedgwood NR3 lots (5,000–7,500 sq ft), a full 900–1,000 sq ft DADU pencils better because tree-protection and rear-yard clearances are easy to hit.

Seattle SDCI + ADUniverse pre-approved plans (see our ADUniverse guide) are the dominant path in 2026 — roughly 70% of our Seattle homeowners choose it. Long-term rents for a 700 sq ft 1BR DADU in Seattle sit at $2,600/mo median in 2026.

Tacoma (Pierce County, WA) is the cash-on-cash winner in Puget Sound in 2026. Turnkey small cottages land at $135K–$165K and rent for $1,450/mo — the best cost-to-rent ratio in the region. Tacoma North End (Proctor, Stadium, Old Town) lots are large (6,000–9,000 sq ft), allowing full DADUs with detached-garage conversions or new-builds without setback headaches.

Tacoma Planning & Development Services runs custom-DADU permits in 10–14 weeks — faster than Seattle custom but slower than Seattle pre-approved. If you're an out-of-state investor buying in Tacoma East Side or South End, the small-cottage path posts 9.1–9.6% cash-on-cash, the highest of any Puget Sound submarket in our 2026 dataset.

Bellevue (King County, WA) builds are the most expensive in Puget Sound — $175K–$330K turnkey depending on size — but rents are also the highest: $3,100/mo for a standard 1BR DADU and $3,550/mo for a 2BR. In Crossroads, Lake Hills, and Wilburton, R-4 and R-5 lots (7,200+ sq ft) easily accommodate a full DADU. In Newport, Somerset, and Cougar Mountain, sloped lots often push you toward a smaller cottage to avoid critical-area review.

Bellevue Development Services runs custom-DADU permits in 12–16 weeks in 2026. Because Bellevue rents are so high, even a $310K full DADU still lands at 6.9–7.4% cash-on-cash — worse than Tacoma on rate, better on absolute monthly cash flow.

Renton (King County, WA) is the fastest large-city permit jurisdiction in 2026 — 8–12 weeks for custom DADUs. Lot sizes in Renton Highlands, Kennydale, and Talbot are typically 6,000–8,500 sq ft, comfortably fitting a 700–900 sq ft full DADU with driveway and utility access. Small cottages ($145K–$175K turnkey) rent for $1,650/mo; full 2BR DADUs ($230K–$275K) rent for $2,600/mo.

Renton also uses a portion of the King County pre-approved DADU catalog for unincorporated pockets adjacent to city limits — worth checking if your lot is in Fairwood, East Renton Plateau, or Skyway.

Kent (King County, WA) looks a lot like Tacoma economically — cottages at $140K–$170K, full DADUs at $220K–$265K. Kent East Hill posts the second-best cash-on-cash in the region (9.0–9.4%) on small cottages because lots are large (7,500+ sq ft), the Kent permit office turns custom DADUs in 8–12 weeks, and rents ($1,600 studio, $2,150 1BR, $2,500 2BR) outrun build cost.

For Kent West Hill and Meridian homeowners with families of aging parents on-site, a full 2BR DADU tends to win on livability. For investor-owners building a first backyard rental, the small cottage is the safer 2026 play.

If your address is technically unincorporated King County (White Center, Skyway, Fairwood, East Renton Plateau) or unincorporated Pierce County (Parkland, Spanaway, Frederickson, Midland), you're not under a city permit desk — you're under the county. King County runs a pre-approved DADU catalog similar to Seattle's ADUniverse, with 5–8 week permit turns. Pierce County custom DADUs run 12–16 weeks in 2026.

The DADU-vs-cottage cost delta ($30K–$60K) and rent delta ($400–$600/mo) hold across county lines — but permit friction is lower in unincorporated King County than inside Seattle for custom builds.

Build a small backyard cottage if: you want the lowest-cost, fastest path to income; you're on a tight budget ($150K–$210K); your lot has setback constraints; you plan owner-occupied STR under Seattle's 2026 rules; or you're rightsizing for aging parents and don't need 2 bedrooms.

Build a full DADU if: you want maximum rental income (long-term or corporate furnished); you need 2 bedrooms for a family renter or multigen use; you want the strongest resale lift; or you plan to eventually unit-lot subdivide and sell separately.

Go pre-approved (ADUniverse in Seattle, King County catalog elsewhere) if: you're building 480–1,000 sq ft on a flat, standard-zone lot and want a 4–6 week permit instead of 4–7 months. About 70% of the Seattle homeowners we work with land here in 2026.