What's the right heating and cooling for an ADU or backyard cottage?
For Seattle ADUs, DADUs, and garage conversions, a single-zone ductless heat pump is the de-facto standard: one compact outdoor unit, one indoor head, full heating and cooling, no ducts, and no load added to the main house's system. Extending the main house's ductwork to a detached backyard cottage almost never pencils out — the runs are long, the balance is wrong, and code expects each dwelling unit to have its own heat source anyway.
The quick answer
- Single-zone ductless is the Seattle ADU standard: compact, quiet, heats and cools
- Extending main-house ducts to a DADU means long runs, code issues, and an unbalanced main system
- Cold-climate units keep full output through any Puget Sound winter — no backup needed
- Typical single-zone market range: $3,500–$6,000 installed (published Seattle-area ranges)
Why ADUs get their own system
A detached cottage obviously can't share ducts — but even attached ADUs and basement conversions do better with independent comfort: their occupancy, schedule, and loads differ from the main house, and code-wise a dwelling unit needs its own heat source. A tenant controlling their own temperature is also simply how renting works.
The space math matters too: ADUs are small by definition, and mini-split heads hang on the wall — no mechanical closet sacrificed to a furnace, no soffits eating an already-tight ceiling.
Seattle's backyard-cottage boom means we've installed ductless in hundreds of DADUs from Ballard to Burien. The pattern is consistent: small footprint, tight envelope, one head covers the living space, and the tenant controls their own comfort without touching the main house's thermostat or electrical load.
The typical ADU install
One wall head sized to the unit's load (small, well-insulated new DADUs often need surprisingly little), the outdoor unit on a pad or brackets outside, a short line-set run, and a dedicated electrical circuit — commonly fed from the ADU's own subpanel. Installs typically run a day. Bathrooms get code-required exhaust; sleeping lofts sometimes warrant a second small head or a floor console.
New-construction DADUs should rope the HVAC conversation in early: head and outdoor-unit placement, line-set routing, and electrical capacity are trivially cheap decisions on paper and annoying ones after drywall.
For attached ADUs and basement conversions, independence matters as much as for detached cottages: different occupancy schedules, different heat loads, and Seattle code expecting each dwelling unit to stand on its own. A shared furnace with dampers and hope is how both units end up uncomfortable.
The multi-zone alternative
If the main house is going ductless anyway, one multi-zone outdoor unit can serve both house and attached ADU — fewer boxes in the yard, one system to maintain. The trade-off: shared fate (one outdoor unit down = both down) and line-set length limits for detached structures. For a detached DADU at distance, independent single-zone usually wins.
Free estimates on new installs should settle this before permit drawings close — moving an outdoor unit or adding a circuit after drywall is the expensive version of a decision that costs nothing on paper during design.
The ADU comfort mistakes
Cheap electric wall heaters 'for now' — the highest-operating-cost heat there is (a heat pump uses ~50% less electricity, per DOE), no cooling, and tenants pay the bill loudly. Undersizing the circuit during construction, then paying an electrician twice. And oversizing the head for a tiny, tight cottage — small spaces are where head-to-load matching matters most.
Also skipped surprisingly often: the rebate check. Ductless heat pumps are mainstays of the utilities' qualified-equipment lists — a new-ADU install can leave real money unclaimed.
In SCL territory, instant discounts ($300–$600 by efficiency tier) apply to qualifying equipment; PSE rebates may apply where the ADU is metered separately. We verify program rules during the estimate because new construction, conversion, and metering setup all change eligibility.
Maintenance is the long game: tenants who never rinse ductless filters are a top callback source in backyard cottages. A simple rinse schedule on the lease checklist prevents noisy heads and lost capacity — professional coil cleaning stays annual, but the mesh filter is monthly in heavy season.
Questions Seattle homeowners ask
Does an ADU need its own permit for HVAC?
Mechanical and electrical permits apply like any install, and ADU projects usually fold HVAC into the broader permit package. We handle permitting on our installs — in Seattle proper and the surrounding King County jurisdictions we work daily. Starting the mechanical conversation before the DADU permit closes avoids the 'we forgot a circuit' change order after drywall.
Is one head really enough for a whole DADU?
For open-plan cottages built to current code — usually yes, comfortably; they're small and tight. Separate sleeping rooms with doors shut are the judgment call: sometimes a second small head or console, sometimes a transfer path. The load calculation settles it honestly. We've heated 400-square-foot backyard cottages with a single wall head through Puget Sound winters without complaint.
What about a basement ADU conversion?
Same logic, one wrinkle: basements run cool and damp in Seattle's long wet season, so heating and dehumidification matter more than cooling. A ductless head handles both (dry mode helps shoulder seasons), and independence from the main furnace keeps both units comfortable and separately controlled. Tenants in basement ADUs especially appreciate controlling their own temperature.
Do ADU heat pumps qualify for rebates?
Qualifying equipment does — City Light instant discounts ($300–$600 by efficiency tier) in SCL territory, PSE rebates where applicable. Terms depend on the situation (new construction vs conversion, metering), so we verify your project against current program rules as part of the free estimate on new installs. Missing rebates on a backyard-cottage project is one of the most common oversights we see — the equipment list and contractor participation matter as much as the hardware choice.
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How we know this
ADU patterns from Glendale installs across Seattle's backyard-cottage boom; efficiency comparisons per U.S. DOE; market ranges from published Seattle-area cost guides (2026); rebate terms verified July 2026.
Reviewed by the Glendale Heating & Air Conditioning team · Last updated 2026-07-15
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