HVAC for ADUs, DADUs, and backyard cottages
The de-facto Seattle ADU standard is a single-zone ductless heat pump: one compact outdoor unit, one head, full heating and cooling, its own controls, no ducts and no burden on the main house's system. The money is made (or lost) at build stage: circuit sizing, head placement, and the rebate check most projects forget. Detached DADUs, attached in-law suites, and backyard cottages all follow the same logic — independent systems for independent dwelling units. Code-built ADUs are small and tight, so loads are modest and one right-sized head usually covers the whole space.
The realities of this stock
- Extending the main house's ductwork almost never pencils out — long runs, code issues, and it unbalances the main system
- ADUs need their own right-sized comfort (and often their own utility metering considerations)
- Space is at a premium — equipment has to be small and quiet
Why extending the main system never pencils
Duct runs to a detached structure are a non-starter, and even attached ADUs do better independent: a dwelling unit needs its own heat source anyway (code), tenants need their own control (sanity), and long duct extensions unbalance the donor system. One right-sized head serves a code-built cottage with room to spare — new DADUs are small and tight, and honest sizing here often surprises people with how little they need.
Seattle's ADU boom means we've sized hundreds of these: a typical 800-square-foot DADU often loads to a single 9,000–12,000 BTU/h head with room to spare. That's a single-zone install, not a multi-zone project — which keeps cost and complexity down.
Decisions that belong at build stage
Rope HVAC into the design early: a dedicated circuit from the ADU subpanel (cheap on paper, annoying after drywall), head placement that respects the sleeping loft, outdoor-unit siting with drainage and neighbor-noise sense, and — for basement conversions — dehumidification thinking, since below-grade spaces run damp (a head's dry mode plus proper bath exhaust covers most cases).
And run the rebate check: ductless heat pumps anchor the utilities' qualified-equipment lists, and new-ADU installs regularly leave SCL/PSE money unclaimed simply because nobody asked. Terms depend on metering and construction type; we verify at estimate.
Shared system vs independent: the real trade-off
Running a multi-zone system from the main house to the ADU saves one outdoor unit — but ties the cottage's comfort to the main house's equipment, maintenance schedule, and failure modes. Independent single-zone systems cost slightly more upfront but give tenants their own thermostat, their own utility meter option, and their own service path.
Seattle's ADU code requires independent heat sources for dwelling units anyway. The design conversation is really about whether the ADU gets its own outdoor unit or shares one with a zone on the main house — and for detached structures, line-set distance limits usually make independent the cleaner answer.
For homeowners building a DADU while also upgrading the main house, phasing matters: ADU HVAC at build stage is cheap to wire; retrofitting a line set through finished landscaping later is not. Get the conduit and pad in during construction even if the outdoor unit install waits.
Landlords: independent systems mean tenant comfort complaints don't become your main-house HVAC problem. A tenant who cranks the ADU thermostat doesn't affect the primary dwelling — worth the modest premium on a separate single-zone over sharing a multi-zone port.
What we actually recommend here
Primary fit
Single-zone ductless heat pump
The de-facto standard for Seattle ADUs: one compact outdoor unit, one head, full heating + cooling, no ducts. Cold-climate models keep full output through any Puget Sound winter.
$3,500 – $6,000 typical Seattle-area range (Seattle Mini Split Pros cost guide, 2026)
Single-zone ductlessAlternate
Multi-zone system shared with the main house
If the main house is going ductless anyway, one multi-zone outdoor unit can serve both — fewer boxes in the yard, one system to maintain.
$6,000 – $15,000 typical Seattle-area range (Seattle Mini Split Pros cost guide, 2026)
Multi-zone systemsDuctless heat pumps are a mainstay of the utilities' qualified-equipment lists — and skipping ductwork avoids what regional installers put at $5,000–$15,000 of retrofit cost.
Every Seattle home is different — this is honest general guidance, not a system spec. The right answer comes from a real in-home load calculation (Manual J), duct and electrical-panel inspection, which is exactly what our free estimate includes. These are typical Seattle-area market ranges compiled from published cost guides (Angi, HomeGuide, and Seattle-area 2025–2026 cost studies) — market context, not a quote from Glendale Heating & Air Conditioning. Your home's size, equipment, ductwork, electrical, and permits change the number. National figures are marked; Seattle-area labor typically runs above national averages.
Questions owners of these homes ask
One head for the whole DADU, even the loft?
Usually — open-plan cottages built to current code carry tiny loads, and a main-space head covers them comfortably. Enclosed sleeping rooms are the judgment call: a transfer path or a second small console where doors stay shut. The room-by-room load calc answers it without drama.
What does ADU comfort cost?
Single-zone market range: $3,500–$6,000 installed (published Seattle-area figures), minus any qualifying utility money. If the main house is going ductless simultaneously, a shared multi-zone can trim total cost — with the shared-fate trade-off the comparison guide covers. Budget the electrical rough-in at build stage even if equipment install waits — adding a circuit after drywall is the expensive version of the same work.
Do ADU heat pumps need their own maintenance plan?
They need the same annual care as any system — and landlords especially value the paper trail (tenant comfort complaints meet a service record). Our plans cover additional equipment at $7/month, which is the tidy way to keep the cottage on the same schedule as the house.
What about ADUs built over garages?
Garage-top ADUs are thermally harsh — three exposed surfaces plus a garage below that swings temperature. A right-sized head handles the load, but placement matters: floor consoles work well in low-ceiling loft bedrooms where wall heads won't fit. Insulation quality at build stage matters more than equipment choice here. We size these with extra margin for the garage thermal bridge — a load calc that treats the unit like interior square footage will undersize the equipment.
Ready for comfort you can count on?
Whether it's a furnace that quit on a cold morning or a whole new system, the Glendale team is ready. Honest pricing, expert work, and 87+ years behind every visit.
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