New Construction and ADU HVAC: What to Budget Before You Build

Most homeowners figure out the HVAC number for a new build or an ADU last, after the framing bid, the roof, the cabinets, and the flooring have already eaten the contingency. That is backwards. Heating and cooling is one of the few line items that gets cheaper when you decide early and more expensive every week you wait, because ducts, line sets, condensate drains, and electrical circuits are nearly free to route through open studs and genuinely painful to retrofit into finished drywall.

Here is how we budget these jobs across Federal Way, Kent, Tacoma, Puyallup, and Olympia, with the real ranges and the trade-offs we'd tell a neighbor.

The short version of the budget

Affluent markets here mean Seattle, Maple Valley, Normandy Park, Black Diamond, Steilacoom, and Fircrest. Luxury means Bellevue, Gig Harbor, and Fox Island. Everything else in our 45-city service area prices as standard.

One honest note on that table: a gas furnace is the cheapest heating-only line item, and it is also the only option that leaves you with no air conditioning and no rebate path. In a new build where you are already paying for the mechanical infrastructure, spending the difference once is almost always smarter than buying cooling separately in three years.

ADUs: the four configurations we actually quote

"ADU" covers wildly different mechanical problems. These are the four we see across Puget Sound.

Detached ADU (DADU)

A standalone structure, usually 400 to 800 square feet, built in the back yard. This is mini-split territory almost every time. A single outdoor condenser with one or two indoor heads handles heating and cooling for the whole footprint, needs no duct chase, and uses the ceiling height you would otherwise give up. Expect the lower half of the $8,000 to $14,000 range for one zone, and the upper half once you are running a head in the bedroom and another in the living space.

Attached ADU or basement conversion

Here the question is whether the main house's existing system has capacity to spare. Usually it does not. A basement ADU has a separate tenant, separate hours, and separate comfort preferences, and splitting a duct run off the main system means two households fighting over one thermostat. A dedicated one- or two-zone ductless system solves that cleanly and keeps utility responsibility separate.

Garage conversion

Insulation first, equipment second. An uninsulated garage is not a load calculation problem, it is a building envelope problem, and putting a heat pump in front of bare studs and an uninsulated roll-up door wastes both money and capacity. Insulate the walls, ceiling, and door, then size the unit.

New primary home

If you are building the house itself, the duct system is being fabricated anyway, so a ducted heat pump is the default. You get one system doing heating and cooling, nothing hanging on the walls, and a mechanical design that matches the home instead of apologizing for it. If you're weighing the two approaches in detail, our mini-split vs. central heat pump breakdown walks the decision line by line.

What the HVAC line item actually includes

When a bid says $13,400, this is what is inside it:

• The outdoor unit and indoor equipment, at the Good, Better, or Best tier you picked
• Refrigerant line set, whip, disconnect, and the pad or bracket
• Condensate management, which is a real cost in a slab or crawlspace build
• Duct connections or, on a full duct job, custom-fabricated commercial-grade sheet metal
• Thermostat or controller, commissioning, and refrigerant charge verification
• The mechanical permit and inspection
• Warranty registration, and the 10-year parts and labor warranty on every install

What it does not include: electrical panel work, insulation, framing changes, or a crane if your site needs one. Ask every bidder what is excluded, not just what is included. That single question sorts good bids from optimistic ones faster than anything else.

The rebate question, answered honestly

This is where new construction differs from everything else we write about. Local utility rebates are generally structured to reward converting an existing heating system to a qualifying heat pump. A brand-new ADU with no prior heat source often has nothing to convert, which means it frequently falls outside the program.

The amount, when a rebate does apply, depends on which utility bills the meter, what is being replaced, and the efficiency of the equipment installed. Rebates follow the meter, not the county line. Varsity reads the utility off your bill, confirms what that utility currently pays, and files the paperwork as part of the job. Programs change and they pay only through their own contractor networks, so we check the current requirements before quoting rather than after.

Also worth saying plainly, because people still plan around it: the federal 25C tax credit ended December 31, 2025. No 2026 installation qualifies. If any bid on your desk includes it, that bid is wrong. For the full picture on what remains, see our Washington rebate guide.

Sizing: the expensive mistake new builds make

Oversizing is the default error in new construction, because a builder's rule of thumb rounds up and nobody gets blamed for "too much capacity." But an oversized heat pump short-cycles, removes less humidity, wears out sooner, and delivers worse comfort than a correctly sized one. In a tight new envelope the load is usually smaller than the square footage suggests, not larger.

A Manual J load calculation is the only honest way to size it, and it weighs insulation values, window area and orientation, ceiling height, air sealing, and layout. It also gets run against the winter design temperature for your specific city, and that varies 12°F across our service area. Olympia, Lacey, Tumwater, DuPont, and Yelm design to 17°F. Puyallup, Sumner, and Graham design to 19°F. Kent and Renton to 21°F. Federal Way, Seattle, and Bellevue to 24°F. Tacoma, Gig Harbor, Lakewood, and Parkland to a mild 29°F. Every Varsity estimate includes a real load calculation at no extra cost.

Electrical, permits, and sequencing

Electrical. A ducted heat pump typically needs a 30 to 40 amp 240V circuit; each ductless zone needs a dedicated 208/230V circuit. An ADU almost always gets its own sub-panel for other reasons, so the HVAC circuit rarely drives the electrical scope by itself. If a main panel upgrade is required, budget roughly $2,000 to $4,500 with a licensed electrician. We are an HVAC contractor, so we specify the circuit and coordinate the scope rather than doing panel work ourselves.

Permits. Washington requires a mechanical permit for essentially every heat pump, mini-split, furnace, or ductwork installation, new construction included. Each of the 45 cities we serve issues its own, or the county does in unincorporated areas. We pull it on every job. No exceptions.

Sequencing. Call us at the framing stage, not at drywall. Rough-in visits let us set duct chases, line set penetrations, condensate routing, and the outdoor unit location while everything is still open, then return for equipment set and commissioning near the end. Most single-zone ADU installs take 4 to 6 hours once the rough-in is done; most whole-home installs finish in one day.

Getting your number

Run the instant estimate tool for Good, Better, and Best starting prices in about 60 seconds, or request a free in-home estimate and we will walk the plans or the site with you. If you want more context on the ductless side first, the Washington mini-split cost breakdown covers pricing by zone count.

Questions about where a system fits in your build? Call (253) 785-4622, Monday through Friday, 8 to 6. Owner-operated out of Federal Way, licensed VARSIHC766RJ, and straight with you about what your project actually needs.