Heating and Cooling an ADU: What Actually Works

Almost every ADU we build in the Puget Sound region ends up on a ductless heat pump, and that is not a style preference — it is what a small, well-sealed building and Washington's energy code push you toward. Here is why, and what it means for comfort and running cost.

A small efficient building barely needs heating

Heating load scales with how much heat a building loses, and a new ADU built to current code loses very little: continuous insulation, good windows and a properly air-sealed envelope. The result is that a 700 square foot unit needs a fraction of the heating output a similarly sized older house would.

That changes the equipment question. A conventional furnace and duct system is oversized, takes up floor area you do not have to spare, and costs more to install than the building needs. The envelope does most of the work — see energy-efficient building in Seattle.

Why ductless heat pumps win

• They heat and cool. One system, both directions. Summers here have stopped being an afterthought, and retrofitting cooling later costs far more than including it now.
• No ductwork. No chases, no soffits, no lost ceiling height — which matters enormously in a small footprint.
• They are efficient. A heat pump moves heat rather than making it, so it delivers several units of heat per unit of electricity. In a mild marine climate like ours they operate in their efficient range nearly all year.
• Zoning is simple. One head serves an open living space; a second can serve a bedroom if the layout needs it.
• The code favours them. Washington has moved decisively toward efficient electric heating, and a heat pump is the straightforward path to compliance rather than something you have to engineer around.

What else goes in

Heating is only half of comfort in a tight building. The other half is air. A well-sealed unit needs deliberate ventilation — typically a quiet continuous exhaust or a heat-recovery ventilator — because the building will not leak enough to ventilate itself. This is a code requirement, not an upgrade, and it is what keeps a small unit from feeling stuffy or damp.

Then the water heater. A compact electric tank or a heat pump water heater is the usual answer; the latter is more efficient but wants a bit of air volume around it, which is a real constraint in a small plan and worth resolving on the drawing rather than on site.

Running cost, honestly

A new, well-insulated ADU on a heat pump is inexpensive to run — the combination of a small conditioned area, a good envelope and efficient equipment is about as good as residential operating cost gets. We will not put a monthly figure on it, because it depends on your rates, your occupant and how warm they like it, and a number invented here would be worth nothing to you.

If the unit is a rental, decide early whether it is separately metered — that decides who pays this bill, and it is a wiring decision made during utility connections, not afterwards. It also feeds the numbers in how much rent an ADU earns.

Where this shows up in your budget

Mechanical is a modest share of an ADU budget next to structure and site work, and it is not a sensible place to economise — it is the system the occupant interacts with every single day. The full breakdown is on realistic costs.

We handle the mechanical design with the build

Equipment selection, ventilation and the energy-code paperwork are part of the permit set, not a decision we leave to you late in construction. See how our process works, or send us your address.