An addition is the one time HVAC gets decided by accident. The framing goes up, the drywall gets scheduled, and somebody asks on a Tuesday whether the new bonus room is going to have heat in it. By then the cheap options are gone.
Here's the honest breakdown we give homeowners in Federal Way, Kent, Puyallup and Olympia who are adding a primary suite, a bonus room over the garage, or a full second-floor build-out.
Start with capacity, not price
The first question is not "what does a duct run cost." It's whether your existing furnace or heat pump has anything left to give.
Every system was sized for the house that existed when it was installed. A 3-ton heat pump sized correctly for an 1,800 square foot rambler does not become a 4-ton system because you framed 400 more square feet onto the back. If the equipment was sized tight — and correctly sized equipment always is — bolting an addition onto it makes the whole house worse. The new room never warms up, and the original bedrooms lose airflow to it.
How to tell if there's room
We answer this with a Manual J load calculation on the whole house including the new space, not a rule of thumb. But three signals tell you a lot before we ever show up:
- The system already struggles. If the upstairs is 6 degrees off in January or the furnace runs nonstop during a cold snap, there is no spare capacity. Full stop.
- The equipment is 12+ years old. Extending ducts into an addition and then replacing the system in three years means paying for the same design work twice.
- The addition is big relative to the house. A 200 square foot sunroom is a different conversation than a 900 square foot second story.
Winter design temperature matters here too. Washington's energy code lists 17°F for Olympia, Lacey and Tumwater and 29°F for Tacoma and Gig Harbor. That twelve-degree spread changes the load on the same floor plan.
Option 1 — Extend the existing ductwork
If the equipment has capacity and the addition sits reasonably close to the existing trunk line, this is usually the cleanest answer.
New ductwork runs $3,000 to $8,000 in standard Washington markets and $4,000 to $10,000 for larger homes, longer runs, or affluent markets. That covers custom-fabricated sheet metal, supply branches, and — the part people forget — a properly sized return. A room with supply air and no return path is a room that pressurizes, pushes air out its own leaks, and never balances.
What drives the price
- Distance from the trunk. Twelve feet across an open crawl space is a different job than routing forty feet around a finished basement and a beam.
- Access. Open framing during construction is the cheapest access you will ever have. Cutting into finished ceilings costs real money and costs it twice, because someone has to patch.
- Trunk capacity. Sometimes the branch is easy but the main trunk is already at its airflow limit. Upsizing it is the honest fix, and it belongs on the estimate up front.
- Crawl space and attic conditions. Puget Sound crawl spaces are damp. New metal gets insulated properly or it sweats and drips.
Option 2 — Add a ductless mini-split zone
When the addition is far from the mechanical room, sits over a slab, or the existing system is maxed out, a dedicated ductless zone is often the smarter buy.
A mini-split runs $8,000 to $14,000 installed in standard markets, with single-zone systems at the low end and multi-head systems near the top. Affluent markets run $9,200 to $16,800. It gives the new space its own thermostat, its own heating and its own cooling, and it doesn't touch the airflow balance of the original house at all.
The trade-offs are real and worth saying out loud. You get a wall head (or a ceiling cassette or slim ducted head, depending on the room), you get a line set running to an outdoor unit, and you get a second system to maintain. What you also get is a room that stays comfortable independent of whatever the rest of the house is doing — which is exactly what an over-the-garage bonus room or a detached-feeling primary suite usually needs.
Each zone needs its own 208/230V circuit. In an addition that's easy, because the electrician is already there.
Option 3 — Replace the system while the walls are open
This is the option nobody prices and a lot of people should.
If your furnace or heat pump is aging, undersized, or you were going to add air conditioning eventually anyway, an addition is the best possible moment to replace it. A ducted heat pump runs $12,000 to $18,000 in standard markets and covers heating and cooling for the whole house, addition included. Sizing the new equipment for the finished floor plan — instead of retrofitting around it later — is how you avoid the classic outcome where the addition works and the original house doesn't.
Done in the same window as the remodel, the duct modification work overlaps with the new install, which usually means one coordinated visit instead of two projects.
Quick comparison
| Approach | Typical installed cost (standard market) | Best when |
|---|---|---|
| ApproachExtend existing ductwork | Typical installed cost (standard market)$3,000 – $8,000 | Best whenEquipment has spare capacity and the addition is near the trunk |
| ApproachAdd a ductless mini-split zone | Typical installed cost (standard market)$8,000 – $14,000 | Best whenAddition is remote, over a slab, or the system is maxed out |
| ApproachReplace system + extend ducts | Typical installed cost (standard market)$12,000 – $18,000 | Best whenEquipment is 12+ years old, undersized, or you want whole-home AC |
| ApproachAdd zoning to an existing system | Typical installed cost (standard market)$2,000 – $5,000 | Best whenDucts reach the space but temperature control is uneven |
Want a starting number before you call anyone? Our instant estimate tool gives Good, Better and Best starting prices in about 60 seconds. For an addition, the real number comes from a walk-through — request a free in-home estimate and we'll look at the framing, the trunk line and the panel together.
Permits, timing and the general contractor
Every one of the 45 cities we serve requires a mechanical permit for new ductwork or new equipment. On an addition, that usually rides along with the building permit your GC already pulled. We pull our mechanical permit, schedule the inspection, and work around the framing and drywall sequence.
Timing, plainly: get the HVAC contractor out at framing. Duct routing, refrigerant line paths, condensate drainage and circuit locations are cheap decisions with the studs open and expensive ones after the walls close. A full duct replacement takes 2 to 4 days; simple modifications are 1 to 2 days and often bundle with equipment installation into one coordinated visit.
About rebates
Be skeptical of anyone promising rebate money on an addition. Local utility heat pump rebate dollars are aimed at converting an existing electric-resistance or fossil-fuel system — not at conditioning new square footage that never had heat. If the project includes replacing the home's existing system, that's a different story and worth checking carefully.
Rebate eligibility also follows the meter, not the county: Tacoma, Fircrest, University Place and Fife are Tacoma Power; Seattle is City Light; Gig Harbor and Fox Island are Peninsula Light. Programs change — we confirm your current eligibility and handle the paperwork. Rebate programs pay only through their own contractor networks and the rules change, so we check the current requirements before quoting rather than after.
The bottom line
An addition doesn't just need heat. It needs heat that doesn't come out of the rest of the house. Run the load calculation first, decide between ducts and ductless second, and price a system replacement as a third option before you assume it's out of reach.
Every Varsity installation carries a 10-year parts and labor warranty, and every estimate includes a real load calculation at no extra cost. Call (253) 785-4622 — tel:+12537854622, Monday through Friday 8 to 6 — or get an estimate online. Bring your framing plans. We'll tell you which lane your project is actually in, even if the answer is "wait until the system needs replacing anyway."