You want a backyard cottage that stays warm through a damp Seattle winter without a big power bill. Is that realistic in a small building? Yes, and the code already pushes new cottages in that direction. Good DADU energy efficiency comes from a tight, well insulated shell first and the right equipment second.

A DADU is a detached accessory dwelling unit, a separate home on the same lot as a house. Details below reflect the official 2021 Seattle Energy Code pages checked in October 2026. Confirm with SDCI for your property.

Why Does DADU Energy Efficiency Start With the Shell?

A small building has a lot of outside surface for its floor area. Walls, roof and floor all lose heat, so every square foot of shell matters more than in a big house. Air leaks around doors and outlets add up fast in a compact space.

The order of decisions matters. Settle the shell first. Size the heating, ventilation and water heating to fit it.

Insulation Levels in the Seattle Energy Code

SDCI publishes the residential chapter of the Seattle Energy Code online. Its Table R402.1.3 lists minimum values for climate zone 5 and marine 4:

• Ceiling insulation of R-60
• Wood frame walls at R-20 plus R-5 continuous, or R-13 plus R-10 continuous
• Floors at R-30
• Slab edge insulation of R-10 to a depth of 4 feet
• Windows with a maximum U-factor of 0.30

These are minimums, not targets. A designer can go further where the budget allows. The roof is often the easiest place to add depth.

How Tight Does the Shell Have to Be?

Section R402.4.1.3.1 caps air leakage for a dwelling unit at 4.0 air changes per hour, measured with a blower door test at 50 pascals. The testing agency signs a written report, and the code says it goes to both the building owner and the code official.

An optional early test at rough-in, while the walls are still open, makes leaks easier to find and seal. The final test is the one on record.

Ask for a copy of that report and keep it with your permit records. It shows how the finished building actually performed, not just how it looked on paper.

What Are Energy Credits?

Beyond the basic shell rules, Section R406 requires each new dwelling unit to earn extra efficiency credits. The number depends on size. A small dwelling unit, under 1,500 square feet of conditioned floor area with less than 300 square feet of windows, needs 5.0 credits. Most other units need 8.0.

Credits come from options such as better windows, a heat pump or a more efficient water heater. Your permit drawings must list which options you chose and the points for each one.

Windows, Doors and Daylight

Windows lose more heat than walls, so their size and quality shape the whole energy picture. A large glass wall facing the yard looks great, but it pushes the design toward more credits or better glazing. The post on window performance explains how to read the labels.

Placement matters too. South glass with a roof overhang can bring in winter sun while limiting summer heat. North windows give steady light with less heat gain.

Heating, Fresh Air and Hot Water

A tight shell cuts the heating load. That lets a modest heat pump do the job in many cottages. The guide to heating and cooling a DADU compares common system types.

A tight cottage needs planned fresh air, since leaks no longer do that work. Read about ventilation systems before you settle the layout. Hot water is the other large load, and the post on water heaters covers the main choices.

A Hypothetical Ballard Cottage

Picture a 700 square foot cottage on a Ballard alley lot. The owner wants big sliding doors facing a shared garden. Its size puts it in the small dwelling category, as long as total window area stays below the 300 square foot limit.

The designer runs the numbers early. Window and door sizes go into a simple spreadsheet next to the floor area.

Two smaller doors instead of one huge slider keep the glass area in check, and a ductless heat pump earns credits while fitting the open plan.

Common Surprises

• Thick walls for continuous insulation eat into floor area on a tight lot.
• Air sealing work happens before drywall and is hard to fix later.
• A large glass wall can change the credit math late in design.
• Recessed lights and plumbing vents in the ceiling need extra sealing work.
• Duct runs through unheated spaces add losses and need careful detailing.

Checklist Before Design Starts

• Estimate conditioned floor area and total window area.
• Choose a wall assembly that meets Table R402.1.3.
• Plan where the air barrier runs at every joint.
• Pick credit options with your designer before drawings are final.
• Ask who will run the blower door test and when.

FAQ

Does a DADU have to meet the same energy code as a house?

Yes. A new detached cottage is new residential construction under the Seattle Energy Code. It must meet the envelope tables, the air leakage cap and the Section R406 credit rules, though a small unit needs fewer credits than a larger house.

Is a blower door test required?

Yes. Section R402.4.1.2 says the dwelling unit shall be tested for air leakage. The test happens after all penetrations of the shell are made, and the signed report goes to you and to the code official.

Can I skip continuous insulation?

Possibly. Table R402.1.3 is the prescriptive path, and the code also allows other compliance paths. Your designer can tell you which path fits your plans and what tradeoffs each one brings.

Where to Begin

Good DADU energy efficiency rarely comes from one big purchase. Treat the shell as the main event and the equipment as the finishing touch. Check your window area against the small dwelling limit early, since that one number can change your credit target. More design topics are collected under design and materials.